ISSN:
0009-2940
Schlagwort(e):
Cyclopropyl building blocks
;
Strained compounds
;
[4 + 2] Cycloaddition
;
Regioselectivity
;
Epoxides, regioselective cleavage of
;
Chemistry
;
Inorganic Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
1-Ethoxy- (8a) and 1-(trimethylsilyloxy)-1,3-pentadiene (8b) cycloadded to 2-chloro-2-cyclopropylideneacetate 6 to give low yields of endo/exo-9a and -9b, respectively. On the other hand, furans 5a-d added 6 (5b and 5d reacted regioselectively) to afford mixtures of the corresponding [4 + 2] cycloadducts endo-11a-d and exo-11a-d in good to high yields. 2-Methyl-5-(trimethylsilyloxy)furan (5e) yielded a mixture of the four cycloadducts endo/exo-11e and endo/exo-12e, which upon attempted purification on silica gel underwent facile hydrolysis to give bicyclic hemiacetals endo/exo-13 and 4-hydroxycyclo-2-hexen-1-ones endo/exo-14, respectively. Similarly endo/exo-11f, and endo/exo-12f, obtained from 2-methoxy-5-methylfuran (5f) and 6, upon hydrolysis in the presence of silica gel gave endo/exo-13 and endo/exo-14 respectively. The structures of endo-14 and exo-12f were established by X-ray crystallography. In the presence of florisil, the epoxides endo-and exo-16 were formed from trimethylsilyloxy-substituted cycloadducts endo/exo-11e. Selective reduction of the α-chlorocarboxylate functionality in the cycloadducts endo-11a-d,f with LiBH4, followed by base-catalyzed cyclization of the resulting chlorohydrins endo-21a-d, f yielded the corresponding epoxides endo-22a-d,f in 22-73% overall yield. Under identical conditions, the cycloadducts exo-11a-f could not be reduced to the corresponding chlorohydrins exo-21a-f. Regioselective reduction of the acetal epoxide endo-22f to the tertiary alcohol endo-23f was achieved with sodium dihydrobis(2-methoxyethoxy)aluminate (Red-Al®). On stirring with moist silica gel, the bicyclic acetals endo-22d,f and endo-23f cleanly hydrolysed to the highly substituted spiro[2.5]oct-6-en-5-ones endo-24d,f (26 and 24% overall yields, respectively, in four steps from 5d,f and 6) and endo-25f, respectively (23% overall yield in five steps from 5f and 6).
Zusätzliches Material:
3 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/cber.19941270614
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