Rasapalli Sivappa

Zinc mediated transesterification of β-ketoesters and coumarin synthesis

The transesterification of ketoesters using zinc and iodine is described. The reaction has been done on a variety of alcohols and phenols. Alcohols furnish transesterified products whereas phenols gave 4-methylcoumarins. The method is highly promising compared with existing methods.

Total synthesis of the putative structure of nagelamide D.

A total synthesis of the putative structure of nagelamide D from imidazole is described. A Stille cross-coupling is used to construct the bis imidazole skeleton, and the pyrrolecarboxamides are introduced via a double Mitsunobu reaction using a pyrrolehydantoin derivative. Discrepancies between the published spectroscopic data and that reported in the literature cast doubts either on the assigned structure or the reported data.

TRANSESTERIFICATION OF KETOESTERS USING AMBERLYST-15

A facile one-to-one transesterification of ketoesters by Amberlyst-15 is described.

Total Synthesis of (±)-Calcaridine A and (±)-epi-Calcaridine A

The first total synthesis of the Leucetta alkaloid calcaridine A is described based on a biosynthetic postulate. Application of an oxidative rearrangement of a 4,5-disubstituted imidazole leads to the formation of both calcaridine A and epi-calcaridine A. An X-ray crystal structure determination on the latter has allowed the assignment of the relative configuration of the epimeric natural product and calcaridine A by extrapolation.

Asymmetric total synthesis of ent-cyclooroidin

An enantiospecific total synthesis of the pyrrole-imidazole natural product cyclooroidin from histidine is described. The key N1-C9 bond is constructed through an intramolecular SN2-type of reaction of a chloro ester. Subsequent imidazole azidation at the 2-position, pyrrole bromination, azide reduction, and deprotection leads to the completion of the synthesis.

Intramolecular Diels–Alder chemistry of 4-vinylimidazoles

An investigation of 4-vinylimidazoles as diene components in the intramolecular Diels-Alder reaction is described. In the course of these studies several parameters affecting the cycloaddition were evaluated including the nature of the imidazole protecting group, the type of dienophile and the linking group. These investigations established that amino linkers were generally more effective than either ethers or esters. In most cases, the cycloadditions were highly stereoselective, resulting in the formation of products derived from an anti transition state. The polysubstituted tetrahydrobenzimidazole core of the pyrrole-imidazole alkaloid ageliferin can be constructed through the use of pseudo dimeric 4-vinylimidazoles.

Total Synthesis of 7′-Desmethylkealiiquinone, 4′-Desmethoxykealiiquinone, and 2-Deoxykealiiquinone

Synthetic approaches to the imidazonaphthoquinone core of kealiiquinone and related Leucetta-derived alkaloids are described. The polysubstituted benzimidazole framework can be constructed through intramolecular Diels–Alder reactions of propiolate-derived enynes followed by oxidation. Adjustment of the oxidation state of the thus formed lactone allows introduction of the 2,3-dihydroxybenzoquinone moiety through a presumed benzoin-like condensation between a phthaldehyde derivative and a masked glyoxal equivalent catalyzed by a cyanide ion. Oxidation of the C2-position can be accomplished through application of an operationally simple treatment of an imidazolium salt with bleach, thus producing the corresponding 2-imidazolone. Debenzylation of a late stage intermediate en route to kealiiquinone was compromised by concomitant O-demethylation upon treatment with triflic acid resulting in the formation of non-natural 7′-desmethylkealiiquinone. Other endgame strategies were evaluated; however, these efforts did not lead to completion of a synthesis of kealiiquinone but did provide access to other closely related analogues.

Facile Syntheses of ABC Ring Skeleton of Camptothecin and Related Alkaloids

Abstract Facile synthetic approaches toward the preparation of ABC ring of camptothecin and related alkaloids starting from cheaper and readily available chemical reagents.

Facile synthesis of a key intermediate for the synthesis of prostanes and isoprostanes

Abstract An unconventional ring opening of β-sultines followed by an elimination step was achieved, leading to the formation of an important synthon for various prostanes and isoprostanes.

Efficient preparation of urocanic acid derivatives from histidine

Urocanic acid derivatives have served as useful starting materials in several total synthesis endeavors in our lab. This paper describes a convenient, large-scale synthesis of several derivatives of urocanic acid via the net elimination of ammonia from histidine.