ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The structure of the cyclo-addition products of ketenes (2) and methylcyclopentadiene could not be predicted, since this ketenophile exists as a rapidly equilibrating 45:54:1 mixture of the 1- (4), 2- (5) and 5-methyl (6) isomers. We studied this reaction using dimethylketene (2a) with a view to monoterpenoid synthesis. Only two (A and B) of the many possible cyclo-adducts were formed in good yield. Using three methods to generate dimethylketene, the ratios of A:B were 90:6, 77:16 and 55:31 respectively.The structures of A and B were proved without making use of the known ketene cyclo-addition rules: Of the many possibilities, all but 7, 8, 9 and 10 were excluded by the C=O and C=C—CH3 IR.-bands and NMR. signals. Structure 7 (filifolone) was rejected by the difference of its NMR. spectrum from those of both A and B, leaving only 8, 9 and 10.For A, structures 9 and 10 were eliminated by two chemical approaches: (1) Peracetic acid oxidation of a mixture rich in A resulted in a combination of Baeyer-Villiger reaction, epoxidation, and epoxide-ketone rearrangement affording a C10-keto-lactone 14, which was cleaved by alkali to acetone and a C7-keto-acid 18. (2) Another peracetic acid treatment produced a keto-epoxide 11, which was converted to a diol 22 and then to a diketo-aldehyde 23 with an NMR. doublet for the aldehyde proton.Of the two structures (9 and 10) left for B, the latter was excluded by the NMR. spectrum of keto-epoxide 15 isolated from the peracid treatment of a mixture rich in B: A decoupling experiment with 15 showed that the -CH2- was placed next to the α-carbon of the carbonyl function.Thus the major cyclo-adduct of dimethylketene and methylcyclopentadiene is 8, and the minor product is 9. This result demonstrates further ketene cyclo-addition specificities on top of the ones expressed by the three known rules. The different ratios of 8 : 9 are considered to be due to varyingly effective competition between the preliminary interconversion of the methylcyclopentadienes and their cyclo-addition of dimethylkctene.A number of other oxidation and reduction products of 8 are described. One of them proved that C7 had migrated in the Baeyer-Villiger reaction.A partial separation of the enantiomers of the major cycloadduct 8 was achieved. By the use of 0.5 equivalent of three chiral primary amines (31, 32, and 33) to form a Schiff base, the excess ketone 8 was in each case distilled off in a chirally enriched form with positive optical rotation. The derived 2.3-dihydroketone 36 was shown to have a positive Cotton effect and, by an application of the octant rule, to have the (1R,5S) chirality orientation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19700530306
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