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Intramolecular [2+2] Photocycloaddition Reactions as an Entry to the 2-oxatricyclo[4.2.1.0(4,9)]nonan-3-one Skeleton of Lactiflorin

Intramolecular [2+2] Photocycloaddition Reactions as an Entry to the 2-oxatricyclo[4.2.1.0(4,9)]nonan-3-one Skeleton of Lactiflorin

Ping Lu, Eberhardt Herdtweck, Thorsten Bach

Chem Asian J. 2012 Aug;7(8):1947-58.

PMID: 22653868

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 (5R*)-3-benzyloxy-5-but-3'-enyl-4-methoxycarbonylfuran-2(5H)-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 (5R*,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 (5R*,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%.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP1171919746	5-((tert-Butyldimethylsilyloxy)methyl)-2,3-dimethoxypyridine		1171919-74-6	Price

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