Premium
Intramolecular [2+2] Photocycloaddition Reactions as an Entry to the 2‐Oxatricyclo[4.2.1.0 4, 9 ]nonan‐3‐one Skeleton of Lactiflorin
Author(s) -
Lu Ping,
Herdtweck Eberhardt,
Bach Thorsten
Publication year - 2012
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201200295
Subject(s) - stereocenter , chemistry , intramolecular force , regioselectivity , stereochemistry , yield (engineering) , butenolide , hydrogenolysis , double bond , enantioselective synthesis , organic chemistry , catalysis , materials science , metallurgy
Abstract Two [2+2] photocycloaddition routes were evaluated as possible ways to access the tricyclic core structure found in the terpene monoglycoside lactiflorin. While the first route via γ‐substituted cyclopentenones was quickly discarded, the reactions of racemic (5 R *)‐3‐benzyloxy‐5‐but‐3′‐enyl‐4‐methoxycarbonylfuran‐2(5 H )‐ones proceeded in high yields and with perfect diastereoselectivity. However, it turned out that the regioselectivity was strongly dependent on the substitution pattern within the but‐3′‐enyl chain, which connects the terminal olefinic double bond to the photoexcited butenolide chromophor. If the chain was unsubstituted or if a tert ‐butyldimethylsilyloxy group was placed at the 2′ position in a syn ‐relationship to the existing stereogenic center (5 R* ,2′ S* ), the crossed product prevailed with regioselectivities of 89:11 to 69:31. If the tert ‐butyldimethylsilyloxy group was positioned at 2′ in an anti ‐relationship to the existing stereogenic center (5 R* ,2′ R* ), the desired straight products were obtained in regioselectivities of 74:24 to 55:45 (61–83 % yield). Following this route, the aglycon part of lactiflorin was obtained by an intramolecular [2+2] photocycloaddition and a subsequent hydrogenolysis in 53 % yield. Its further conversion into the natural product after glycosylation included a methyl addition to the lactone carbonyl group, which was optimized to give the desired key intermediate in a yield of 70 %. The further conversion to lactiflorin was achieved in four steps and with an overall yield of 49 %.