Don M. Coltart

On the power of chemical synthesis: Immunological evaluation of models for multiantigenic carbohydrate-based cancer vaccines

Synthetic carbohydrate cancer vaccines have been shown to stimulate antibody-based immune responses in both preclinical and clinical settings. The antibodies have been observed to react in vitro with the corresponding natural carbohydrate antigens expressed on the surface of tumor cells, and are able to mediate complement-dependent and/or antibody-dependent cell-mediated cytotoxicity. Furthermore, these vaccines have proven to be safe when administered to cancer patients. Until recently, only monovalent antigen constructs had been prepared and evaluated. Advances in total synthesis have now enabled the preparation of multivalent vaccine constructs, which contain several different tumor-associated carbohydrate antigens. Such constructs could, in principle, serve as superior mimics of cell surface antigens and, hence, as potent cancer vaccines. Here we report preclinical ELISA-based evaluation of a TF–Ley–Tn bearing construct (compound 3) with native mucin glycopeptide architecture and a Globo-H–Ley–Tn glycopeptide (compound 4) with a nonnative structure. Mice were immunized with one or the other of these constructs as free glycopeptides or as keyhole lymphet hemocyanin conjugates. Either QS-21 or the related GPI-0100 were coadministered as adjuvants. Both keyhole lymphet hemocyanin conjugates induced IgM and IgG antibodies against each carbohydrate antigen, however, the mucin-based TF–Ley–Tn construct was shown to be less antigenic than the unnatural Globo-H–Ley–Tn construct. The adjuvants, although related, proved significantly different, in that GPI-0100 consistently induced higher titers of antibodies than QS-21. The presence of multiple glycans in these constructs did not appear to suppress the response against any of the constituent antigens. Compound 4, the more antigenic of the two constructs, was also examined by fluorescence activated cell sorter analysis. Significantly, from these studies it was shown that antibodies stimulated in response to compound 4 reacted with tumor cells known to selectively express the individual antigens. The results demonstrate that single vaccine constructs bearing several different carbohydrate antigens have the potential to stimulate a multifaceted immune response.

Direct Carbon-Carbon Bond Formation via Soft Enolization: A Biomimetic Asymmetric Mannich Reaction of Phenylacetate Thioesters

An asymmetric Mannich reaction of phenylacetate thioesters and sulfonylimines using cinchona alkaloid-based amino (thio)urea catalysts is reported that employs proximity-assisted soft enolization. This approach to enolization is based on the cooperative action of a carbonyl-activating hydrogen bonding (thio)urea moiety and an amine base contained within a single catalytic entity to facilitate intracomplex deprotonation. Significantly, this allows thioesters over a range of acidity to react efficiently, thereby opening the door to the development of a general mode of enolization-based organocatalysis of monocarboxylic acid derivatives.

Copper(I)-Catalyzed Addition of Grignard Reagents to in Situ-Derived N-Sulfonyl Azoalkenes: An Umpolung Alkylation Procedure Applicable to the Formation of Up to Three Contiguous Quaternary Centers

The alpha-alkylation of N-sulfonyl hydrazones via in situ-derived azoalkenes provides an umpolung approach to ketone alpha-alkylation that has considerable potential with regard to catalysis and the direct incorporation of functionality not amenable to the use of enolate chemistry. Herein, we describe the first Cu(I)-catalyzed addition of Grignard reagents to in situ-derived N-sulfonyl azoalkenes. This method is remarkable in its ability to deliver highly sterically hindered compounds that would be difficult or impossible to synthesize via traditional enolate chemistry, including those having up to three contiguous quaternary centers.

Development of a Strategy for the Asymmetric Synthesis of Polycyclic Polyprenylated Acylphloroglucinols via N-Amino Cyclic Carbamate Hydrazones: Application to the Total Synthesis of (+)-Clusianone

A broadly applicable asymmetric synthetic strategy utilizing N-amino cyclic carbamate alkylation that provides access to the various stereochemical permutations of a common structural motif found in many polycyclic polyprenylated acylphloroglucinols is described. The utility of this methodology is demonstrated through the first asymmetric total synthesis of the antiviral agent (+)-clusianone.

Simple and Efficient Asymmetric α‐Alkylation and α,α‐Bisalkylation of Acyclic Ketones by Using Chiral N‐Amino Cyclic Carbamate Hydrazones

Ketone a-alkylation is fundamental to organic synthesis. Remarkably, however, only one effective asymmetric version of this transformation applicable to acyclic systems is available. Introduced over 25 years ago, this method is based on the alkylation of metalated SAMP/RAMP hydrazones, and has enabled numerous total syntheses. Unfortunately, its further development has been impeded as a result of certain inherent limitations. For instance, the dialkyl hydrazones used are only weakly acidic, so azaenolate formation requires exposure to lithium diisopropylamide (LDA) for 2–10 h. Alkylation must then be conducted at an extremely low temperature ( 110 to 78 8C), making large-scale applications impractical. Moreover, removal of the costly auxiliary under recommended conditions (ozonolysis or quaternization/hydrolysis) limits functional group compatibility. The auxiliary itself is liberated in an altered form that hinders recycling. Given these limitations, it is apparent that asymmetric a-alkylation of ketones remains an unsolved problem. Herein, we report a substantial advance in this field through the development of chiral N-amino cyclic carbamates (ACCs). These auxiliaries significantly diminish the drawbacks associated with the use of chiral dialkyl hydrazines, yet still provide excellent stereoselectivity. In addition, the auxiliaries exhibit a unique directing effect that overrides the inherent selectivity of LDA, thus enabling the asymmetric a,a-bisalkylation of ketones, a previously unattainable transformation. Hydrazones possessing an electron withdrawing group (1, Scheme 1), which we term activated hydrazones, are readily formed from the corresponding substituted hydrazines (e.g., hydrazides, sulfonyl hydrazides, etc.) and ketones under mild conditions, and are rapidly hydrolyzed under similarly mild conditions, making them excellent candidates for auxiliarybased synthetic methods. We anticipated that the enhanced acidity of these activated hydrazones would enable rapid metalation over a range of temperatures. Moreover, the substantial electron density imparted to the electron withdrawing group in the derived azaenolates (2) should lead to tight metal cation binding, in a manner analogous to, for example, chelation of hydroxamate anions. In the context of asymmetric transformations, this could potentially bring high facial selectivity during alkylation, even at temperatures well above 110 to 78 8C, as required of SAMP/RAMP systems. Collectively, these factors suggested that chiral hydrazines substituted with a conjugated electron-withdrawing group could provide the basis of a simple method for asymmetric ketone a-alkylation. We focused our initial studies along these lines on the easily accessible ACCs. Thus, 3 was prepared by amination of the corresponding oxazolidinone and was then condensed with 3-pentanone to give 8 (Table 1). This activated hydra-

Construction of carbohydrate-based antitumor vaccines: synthesis of glycosyl amino acids by olefin cross-metathesis

Abstract The synthesis of biologically relevant glycosyl amino acids from corresponding O-allyl glycosides is described. The procedure involves a cross-metathesis reaction with Fmoc- l -allylglycine benzyl ester, followed by reduction of the resulting olefin via catalytic hydrogenation, with the concomitant release of the free acid. This method has also been applied to the breast and prostate cancer antigen Globo-H, to afford a hexasaccharide glycosyl amino acid that has been previously incorporated in a polyvalent antitumor vaccine.

Direct carbon-carbon bond formation via chemoselective soft enolization of thioesters: a remarkably simple and versatile crossed-Claisen reaction applied to the synthesis of LY294002.

Thioesters undergo chemoselective soft enolization and acylation by N-acylbenzotriazoles on treatment with MgBr2 x OEt2 and i-Pr2NEt to give beta-keto thioesters. Prior enolate formation is not required, and the reaction is conducted using untreated CH2Cl2 open to the air. The coupled products are stable synthetic equivalents of beta-keto acids and can be converted directly into beta-keto esters, beta-keto amides, and beta-diketones under mild conditions. The utility of this carbon-carbon bond-forming method is shown through the synthesis of the PI3-K inhibitor LY294002.

Asymmetric total synthesis of (+)- and (―)-clusianone and (+)- and (―)-clusianone methyl enol ether via ACC alkylation and evaluation of their anti-HIV activity

The total asymmetric synthesis of (+)- and (-)-clusianone and (+)- and (-)-clusianone methyl enol ether is reported. Asymmetric induction is achieved through the use of ACC alkylation, providing the key intermediates with an er of 99:1. The four synthetic compounds were evaluated for their anti-HIV activity. Both (+)- and (-)-clusianone displayed significant anti-HIV activity.

Origins of Stereoselectivity in the α-Alkylation of Chiral Hydrazones

Density functional theory calculations and experiment reveal the origin of stereoselectivity in the deprotonation-alkylation of chiral N-amino cyclic carbamate (ACC) hydrazones. When the ACC is a rigid, camphor-derived carbamate, the two conformations of the azaenolate intermediate differ in energy due to conformational effects within the oxazolidinone ring and steric interactions between the ACC and the azaenolate. An electrophile adds selectively to the less-hindered π-face of the azaenolate. Although it was earlier reported that use of ACC auxiliaries led to α-alkylated ketones with er values of 82:18 to 98:2, B3LYP calculations predict higher stereoselectivity. Direct measurement of the dr of an alkylated hydrazone prior to removal of the auxiliary confirms this prediction; the removal of the auxiliary under the reported conditions can compromise the overall stereoselectivity of the process.

Origins of Stereoselectivity in the alpha-Alkylation of Chiral Hydrazones

Density functional theory calculations and experiment reveal the origin of stereoselectivity in the deprotonation-alkylation of chiral N-amino cyclic carbamate (ACC) hydrazones. When the ACC is a rigid, camphor-derived carbamate, the two conformations of the azaenolate intermediate differ in energy due to conformational effects within the oxazolidinone ring and steric interactions between the ACC and the azaenolate. An electrophile adds selectively to the less-hindered π-face of the azaenolate. Although it was earlier reported that use of ACC auxiliaries led to α-alkylated ketones with er values of 82:18 to 98:2, B3LYP calculations predict higher stereoselectivity. Direct measurement of the dr of an alkylated hydrazone prior to removal of the auxiliary confirms this prediction; the removal of the auxiliary under the reported conditions can compromise the overall stereoselectivity of the process.