Studies on the stereo- and regiochemistry of [4+2] cycloaddition of nitroso compounds to chiral halobenzenediols. Total synthesis of lycoricidine

Abstract

All four monosubstituted halobenzenes Sa-d have been converted to the corresponding cis-arenediols by a microbial oxidation with the bacterium Pseudomonas putida strain 39- D. Enantiomerically pure cis-diols were obtained from chloro- and bromobenzene. The acetonides of these two diols have been utilized as useful synthons in the preparation of several natural products, like conduritols E 1 and F 2, aminoconduritol A-1 3 and lycoricidine 4.

The optical purity of bromocyclohexadienediol, was determined by its conversion to three different natural products ((+)-conduritol E 1, (-)-conduritol F 2, and (+)- aminoconduritol A-1 3), and compared with their optical rotation.

Several dienophiles were added to the protected cis-arenediols to study the regio- and stereochemistry of the cycloaddition. The addition of dienophiles to the acetonide of the diols was anti in every case. When ethyl propiolate was used, two regioisomers 7 and 8 were obtained. In contrast, the addition of nitroso dienophiles, derived in situ from the oxidation of hydroxamic acids, to protected halocyclohexadienediols provided chiral bicyclic oxazines of type 9, as single isomers. These compounds were converted by reductive cleavage to 1,4-hydroxy amides. The anti-addition to the acetonide and the stereospecific formation of the two new chiral centers adjacent to the acetonide was exploited in an approach to a more complex phenanthrene natural product lycoricidine 4.