Henryk Krawczyk

Organocatalytic asymmetric synthesis of organophosphorus compounds.

240 Years have passed since the discovery of elemental phosphorus. During that time organophosphorus chemistry has emerged as an interesting and exciting field of research. Recently organophosphorus chemistry has been raised to a new level. Organophosphorus compounds have found applications in asymmetric organocatalysis for the synthesis of optically active compounds of synthetic or biological importance. The aim of this review article is to present recent contributions to this developing field of chemistry and to point out synthetic advantages of methodologies developed so far.

Organocatalytic Domino Michael–Knoevenagel Condensation Reaction for the Synthesis of Optically Active 3‐Diethoxyphosphoryl‐2‐oxocyclohex‐3‐enecarboxylates

Versatile dominoes: A novel, organocatalytic, Michael-Knoevenagel condensation domino reaction of ethyl 4-diethoxyphosphoryl-3-oxobutanoate with various aryl- and aliphatic-substituted alpha,beta-unsaturated aldehydes catalyzed by a chiral diarylprolinol ether has been successfully performed. The reaction proceeds in a highly enantio- and diastereoselective manner giving access to optically active 6-substituted-3-diethoxyphosphoryl-2-oxocyclohex-3-enecarboxylates (see scheme).A novel, organocatalytic, highly enantio- and diastereoselective synthetic approach towards optically active 6-substituted-3-diethoxyphosphoryl-2-oxocyclohex-3-enecarboxylates is presented. Our methodology utilizes a Michael-Knoevenagel domino reaction sequence of ethyl 4-diethoxyphosphoryl-3-oxobutanoate and alpha,beta-unsaturated aldehydes catalyzed by a chiral diarylprolinol ether. The cyclohexenecarboxylates obtained are particularly well suited for the preparation of highly functionalized cyclohexene and cyclohexane derivatives, with up to four chiral centers and high levels of stereocontrol.

Enantioselective Organocatalytic Approach to α-Methylene-δ-lactones and δ-Lactams

We present the first enantioselective organocatalytic approach for the synthesis of alpha-methylene-delta-lactones and delta-lactams. Our methodology utilizes the Michael addition of unmodified aldehydes to ethyl 2-(diethoxyphosphoryl)acrylate as the key step affording highly enantiomerically enriched adducts, which can be transformed into the target compounds maintaining the high stereoselectivity achieved in the first step. This methodology has been shown to be general and various optically active gamma-substituted alpha-methylene-delta-lactones and delta-lactams can be easily accessed.

Synthesis and biological evaluation of α-methylidene-δ-lactones with 3,4-dihydrocoumarin skeleton

A series of new 3-methylidenechroman-2-ones bearing various aromatic moieties and various substituents at position 4 were synthesized in a three step reaction sequence. Friedel-Crafts alkylation of phenols or naphthols using ethyl 3-methoxy-2-diethoxyphosphorylacrylate in the presence of trifluoromethanesulphonic acid gave 3-diethoxyphosphorylchromen-2-ones. These compounds were employed as Michael acceptors in the reaction with Grignard reagents to give adducts which were finally used as Horner-Wadsworth-Emmons reagents for the olefination of formaldehyde. All obtained 3-methylidenechroman-2-ones were tested against two human leukemia cell lines NALM-6 and HL-60 as well as MCF-7 breast cancer and HT-29 colon cancer adenocarcinomas. Several obtained methylidenechromanones displayed high cytotoxic activity with IC(50) values below 1 μM, mainly against leukemia and MCF-7 cell lines. Investigation of structure-activity relationships revealed that the presence of additional, ortho-fused benzene ring and n-butyl or i-propyl group in position 4 enhances the activity. Selected methylidenechromanones were also tested on normal human umbilical vein endothelial cells (HUVEC) and chromanone 14o was found to be eightfold more toxic against MCF-7 than normal cells. Furthermore, antimicrobial assays revealed that chromanone 14n is highly active and bactericidal at concentration equal to MIC or 2MIC against nosocomial and community-associated staphylococci (MRSA) which are resistant to most or all available therapeutic classes of antimicrobial drugs.

Synthesis and cytotoxic evaluation of β-alkyl or β-aryl-δ-methyl-α-methylene-δ-lactones. Comparison with the corresponding γ-lactones

We present a simple and general strategy for the synthesis of beta,delta-disubstituted-alpha-methylene-delta-lactones starting from easily available tert-butyl 2-(diethoxyphosphoryl)alk-2-enoates. The elaborated synthetic protocol includes pyrrolidine-catalyzed Michael addition of acetone, diastereoselective reduction of the carbonyl group, lactonization and finally the Horner-Wadsworth-Emmons reaction with formaldehyde. All alpha-methylene-delta-lactones were evaluated in vitro against mouse leukemia cell line L-1210 and two human leukemia cell lines HL-60 and NALM-6. Comparison of cytotoxic activity with corresponding alpha-methylene-gamma-lactones is also discussed.

Nitroalkanes as nucleophiles in a self-catalytic Michael reaction

A simple, effective procedure for the preparation of 4-nitroalkanoates 6–9 by the Michael reaction of nitroalkanes 2–5 with the acrylate 1 is described. The primary nitro adduct 6 undergoes isomerization to hydroxamic acid 10 while heated in boiling nitromethane. Consecutive reactions of the latter compound lead to the formation of N-hydroxysuccinimide 11 and its N-ethoxy derivative 12. The spontaneous Nef reaction of the mother 4-nitrobutanoic acid 15 gives N-hydroxysuccinimide 14. The analogous reaction of secondary nitroalkanoic acids 16 and 17 provides 4-oxoalkanoic acids 18 and 19, respectively. Intramolecular participation by the carboxylic acid group in the Nef reaction is proposed.

An Efficient Synthesis of β,γ -Disubstituted α -Diethoxyphosphoryl-γ -lactones: A Convenient Approach to α -Methylene-γ -lactones

A synthesis of β,γ -disubstituted α -diethoxyphosphoryl-γ -lactones from easily available dicyclohexylammonium 3-aryl-2-diethoxyphosphoryl-4-oxopentanoates is reported. The phosphonolactones were transformed into the corresponding α -methylene-γ -lactones by means of the Horner–Wadsworth–Emmons reaction with formaldehyde.

Three-component reaction of 3-(diethoxyphosphoryl)coumarin, enolizable ketones and primary amines: Simple, stereoselective synthesis of benzo[1,3]oxazocine skeletons

The synthesis of benzoxazocine phosphonates was accomplished by means of the three-component reaction of 3-(diethoxyphosphoryl)coumarin, enolizable ketones and benzylamine. An asymmetric version of this reaction was also developed. Treatment of the coumarin with cyclopentanone and (R)- or (S)-1-phenylethylamine afforded enantiomerically pure benzoxazocine phosphonates in a 2.5:1 diastereoisomeric ratio. In contrast, similar reaction with cyclohexanone provided α-phosphono-δ-enamides as single diastereoisomers. The mechanistic features of the reaction were discussed. A transition state model rationalizing its stereochemical outcome was proposed. The α-phosphono-δ-lactams obtained were transformed into the corresponding α-methylene-δ-lactams. The cytotoxicity of these compounds was evaluated.

X-ray investigations of bicyclic α-methylene-δ-valerolactones. V. (4aS,7R,8aR)-7-Isopropenyl-4a-methyl-3-methyleneperhydrochromen-2-one

The title compound, C(14)H(20)O(2), adopts a conformation in which the delta-valerolactone and cyclohexane rings are almost coplanar with one another. The gamma-methyl substituent occupies an axial position with respect to the cyclohexane ring. The delta-valerolactone moiety adopts an envelope arrangement, while the cyclohexane ring exists in a chair conformation.

X-ray investigations of bicyclic alpha-methylene-delta-valerolactones. III. trans-(4aR,8aR)-4a-Methoxy-3-methyleneperhydrochromen-2-one.

The title compound, C11H16O3, adopts a conformation in which the delta-valerolactone and cyclohexane rings are almost coplanar with one another. The beta-methoxy substituent occupies an axial position with respect to the cyclohexane ring. The delta-valerolactone moiety adopts a half-chair arrangement, while the cyclohexane ring exists in a chair conformation.