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Glycosylidene Carbenes Part 12 . A new synthesis and some reactions of spirooxiranes
Author(s) -
Vasella Andrea,
Dhar Preeti,
Witzig Christian
Publication year - 1993
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19930760433
Subject(s) - chemistry , yield (engineering) , diazirine , thermal decomposition , cyclohexanone , photodissociation , carbene , medicinal chemistry , enol , acetone , cyclopentanone , enol ether , steric effects , decomposition , ketone , isobutyraldehyde , organic chemistry , photochemistry , catalysis , materials science , metallurgy
The diazirine 1 , upon thermolysis or photolysis in either acetone or cyclohexanone, at different concentrations, yield the spiro epoxides 2 and 3 , and 4 and 5 , respectively ( Scheme 1 ). Yield of 2 and 3 depended both on the temperature and the concentration, and correlated inversely with the yield of the major by‐product, the enol‐derived glycoside 6 . Other by‐product were the benzyloxglycal 7 and the lactone azines 8 . ZnCl 2 ‐Promoted methanolysis of 2 under mild condition yielded mixture of the uloside 9 and 10 (1.2:1); similarly, 4 yielded 11 and 12 (1.8:1; Scheme 2 ). More strongly acidic conditions converted 11 into 12 , evidencing that ZnCl 2 ‐promoted methanolysis proceeds under kinetic control, which is rationalized. The diazirine 13 , upon thermolysis of Photolysis in either acetone of cyclohexanone, yielded the α‐ D ‐configurated spiro epoxides 14 and 16 , and the α‐ D ‐configurated dihydrooxazoles 15 and 17 , respectively ( Scheme 3 ), which are either formed by ring‐opening of ß‐ D ‐epoxides, by competitive interception of the initially formed, hypothetical addition products of the intermediate carbene to the ketones. The glycosylidene carbenes, derived from 1 or 13 are not very reactive towards ketones, yields are good only when sterically unhindered ketones are used in large excess.