J. Org. Chem. 1993,58, 3169-3171
Ozonolysis of Cyclic Enol Ethers: An Efficient Strategy to Aldol and Homoaldol Compounds
3169
1) 03 / CH&
I MeOH
2) Et3N / A c 2 0 or DMS
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R4 A3
Stephan Hillers, Anja Niklaus, and Oliver Reiser' Znstitut fur Organische Chemie, Universitat Gdttingen, Tammannstr. 2, 3400 Gdttingen, Germany
of 8-unsubstituted enol ethers to create a stable este9 rather than an aldehyde functionality. Therefore, the dihydropyrans Id-f, as well as the dihydrofurans 3 were Receiued November 24, 1992 ozonolyzedin a dichloromethane/methanolmixture under conditions developed by Schreiberg for carbocycles. According to this procedure the hydroperoxides, which are In our ongoing studies to develop asymmetric, catalytic generated in situ, are dehydrated with triethylamine/acetic methodology to functionalized acyclic compounds, we anhydride after azeotropic removal of methanol with envisioned substituted cyclic enol ethers 1 as direct benzene. These conditions could be successfully applied precursors to aldol and homoaldol products. Since a to cyclic enol ethers; however, it was discovered that the variety of catalytic,l as well as noncatalytic,2 methods to removal of methanol can be omitted with equally good enantiomerically enriched dihydrofurans and dihydroresults. This variation is especially useful for large-scale pyrans are known, we investigated the possibility of their preparations,since the hydroperoxides which result from oxidative ring opening by ozonolysis to develop a new the ozonolysis of e g . 3a are very explosive and should access to aldol3 and homoaldo14products. only be handled at low temperatures. Thus, the 3-formyAlthough enol ethers play an important role in synthetic chemistry, their ozonolysis has not widely been ~ t u d i e d . ~ loxy ester 4a was conveniently prepared by this protocol (entry 10)as the sole product. Also, 3,4-dihydro-6-phenylIn particular, there are only a few accounts on the 2H-pyran ( l e ) (entry 8) gave rise to the 4-formyloxy ester ozonolysis of cyclic enol ethers.6 Nevertheless, Danishef2e in 91 % isolated yield accompanied by minor amounts sky7 et al. have noted that dihydro-y-pyrones can be (
