2‐Stannyl‐1, 3‐dithiane Herstellung, Sn/Li‐Transmetallierung und Verwendung für Cyclisierungen

Abstract

2‐Stannyl‐1,3‐dithianes. Preparation, Sn/Li‐Transmetallation, and Use for CyclizationsIn order to test the possibility of generating nucleophilic 2‐lithio‐1, 3‐dithiane centers in the presence of electrophilic groups in the same molecule, the stannylated dithianes 1‐3 were prepared or generated. Solutions of the lithio derivatives 2a and 2b could either be obtained by metallation of 1 with lithiumdiisopropylamide (LDA) or by transmetallation of 3 with alkyllithium reagents. Alkylations of 2 led to the alkyl‐stannyl‐dithianes 4‐7. Additions of the trimethylstannylated lithiodithiane 2a to aldehydes and ketones at low temperature led ‐ after hydrolysis ‐ to the adduct alcohols 8; warming up to room temperature before hydrolysis furnished keten thioacetals 9 only with acetone (→ 9b) and cyclohexanone (→ 9c) as carbonyl component, while still the simple adducts 8a and 8d were isolated with benzaldehyde and cyclohexenone, respectively. Methyl benzoate and benzoic acid anhydride reacted with 2a to produce the tin‐free derivatives 12 and 14, respectively.It is shown that the Sn/Li‐exchange at the 2‐position of dithianes 4‐7, 15 and 16 takes place within minutes at −78°, whereas H/Li‐metallation does not occur at all at this temperature. In situ preparation of the cyclization products 17‐19 from halo‐epoxides is described. The overall yields of Sn/Li‐exchange (3 → 2), epoxyalkylation (2 → 15 and 16, repectively), Sn/Li‐exchange in 15and 16, cyclization (→ 17–19) are twice as high (up to 80%) with the tributylin than with the trimethyltin derivatives. The intramolecular 1, 3 nucleophilic reaction 20a → 17 is complete within 5 min at −78°. The total yields of cyclization products by the tin route (3b → 16 → 20b → 18 + 19) and by direct metallation (1, 3‐dithiane →21b → 20b → 18 + 19) are 63 and 30%, respectively.