Facial selectivity in the reactions of 1,3-dipoles with cis- and trans-3,4-dimethyl-1-methoxycarbonyl cyclobutenes
Face selectivity of the regiospecific reactions of 1,3-dipoles [diazomethane ( 5), 3,4-dihydroisoquinoline-N-oxide ( 6) and nitrile oxides ( 7)] with trans-3,4-dimethyl-1-methoxycarbonyl cyclobutene ( 4) is controlled by steric factors with dominant formation of the anti diastereoisomer (anti/syn: 8...
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| Published in: | Tetrahedron Vol. 53; no. 27; pp. 9279 - 9292 |
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| Main Authors: | , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Elsevier Ltd
07-07-1997
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| Online Access: | Get full text |
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| Summary: | Face selectivity of the regiospecific reactions of 1,3-dipoles [diazomethane (
5), 3,4-dihydroisoquinoline-N-oxide (
6) and nitrile oxides (
7)] with trans-3,4-dimethyl-1-methoxycarbonyl cyclobutene (
4) is controlled by steric factors with dominant formation of the anti diastereoisomer (anti/syn: 82:18 for
5, 54:46 for
6 and ≈ 74:26 for
7) through the sterically less crowded transition state. On passing to cis-3,4-dimethyl-1-methoxycarbonyl cyclobutene (
3b) the expected increase in diastereoselectivity is observed only in the case of the reaction of
3b with 3,4-dihydroisoquinoline-N-oxide (anti/syn = 94:6) while face selectivity decreases in the reaction of
3b with diazomethane (anti/syn = 72:28) and there is a reversal of face selectivity in the reactions of
3b with nitrile oxides (anti/syn ≈ 42:58). In the latter reaction the dominant syn diastereoisomer is formed via the sterically more congested TS. A syn orienting electronic effect (not yet clearly identified) is evidently present in the reactions of
3b with 1,3-dipoles and in the reaction of
3b with nitrile oxides it is strong enough to overcome relevant anti orienting steric effects.
The reactions of diazomethane and 3,4-dihydroisoquinoline-N-oxide with
3b afford syn/anti mixtures (28:72 and 6:94, respectively) in which the syn adduct deriving from the sterically less congested TS is dominant. In contrast, the sterically more crowded syn TS is slightly favored over its anti counterpart (syn:anti ≈ 58:43) in the reaction of nitrile oxides with
3b. The facial selectivity of the reactions of all these 1,3-dipoles with the trans isomer of
3b is controlled by steric factors. |
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| ISSN: | 0040-4020 1464-5416 |
| DOI: | 10.1016/S0040-4020(97)00539-5 |