Stereoselective intramolecular copper(I)--catalyzed [2 + 2]-photocycloadditions. Enantioselective synthesis of (+)- and (-)-grandisol

Article abstract of DOI:10.1021/jo00127a034

This work deals with copper(I)-catalyzed intramolecular [2 + 2]-photocycloadditions of 1,6-diene derivatives. The bridgehead carbons C-1 and C-5 of the resulting bicyclo[3.2.0]heptanes are generated stereoselectively by using chiral starting material, chiral catalysts, or chiral auxiliaries. The irradiation of (S)-3 leads to enantiomerically pure 4 and 5 which opens a new synthetic route to enantiomerically pure (+)- and (-)-grandisol 9. The use of chiral copper complexes as catalysts delivers enantiomeric excesses below 5%. The reason for these small excesses is a low reactivity of the chiral copper complexes, as confirmed by CD-spectroscopic measurements. Malic acid or amino carboxylic acid derivatives as chiral auxiliaries yield the bicyclic alcohols 4 and 5 with enantiomeric excesses up to 15%. The employment of a chiral diol as an auxiliary delivers a chiral ketal 36, and the resulting ketone 7 exhibits enantiomeric excesses up to 60%.