Pyran, dihydroSubject

Subjected to similar conditions, (35,65)-3,6-dihydro-6-methoxy-7V,7V-dimethyl-2//-pyran-3-ac-etamide is converted to (3af ,65,7S,7a/ )-hexahydro-7-iodo-6-methoxy-2//-furo[3,2-c]pyran-2-... 

Since the disclosures that the thermal dimerizations of acrolein and methyl vinyl ketone provide the 3,4-dihydro-2//-pyrans (1, 2) derived from 4ir and 2Tt participation of the a,3-unsaturated carbonyl compound in a Diels-Alder reaction, an extensive series of related observations have been detailed. This work has been the subject of several comprehensive reviews - - including the Desimoni and Tacco-ni extensive tabular compilation of work through 1974. Consequently, the prior reviews should be consulted for thorough treatments of the mechanism, scope, and applications of the [4 + 2] cycloaddition reactions of a,3-unsaturated carbonyl compounds. The [4 + 2] cycloaddition reactions of 1-oxa-1,3-butadienes with their 4-it participation in the Diels-Alder reaction exhibit predictable regioselectivity with the preferential or exclusive formation of 2-substituted 3,4-dihydro-2W-pyrans (equation 1). The exceptions to the predicted regioselectivity that have been observed involve the poorly matched [4 + 2] cycloaddition reaction of an electron-deficient l-oxa-l,3-butadiene with an electron-deficient dienophile, e.g. methyl crotonate or methacrolein. - Rigorous or simplified theoretical treatments of the [4 + 2] cycloaddition reaction of 1-oxa-1,3-butadienes predict the preferential formation of 2-substituted 3,4-dihy-dro-2f/-pyrans and accommodate the preferred endo approach of the reactants in which the carbon-carbon bond formation is more advanced than carbon-oxygen bond formation, i.e. a concerted but nonsynchronous [4 + 2] cycloaddition reaction. ... 

In the development of new sugar amino acids and peptidomimetics, 6-allyloxy-2-[(terr-butyl diphenylsilanyloxy)methyl]-3,6-dihydro-27/-pyran-3-ol 189 was subjected to the refluxing... 

Since the thermal dimerizations of acrolein and methyl vinyl ketone were shown to provide the 3,4-dihydro-2//-pyrans l,12 an extensive range of related observations have been disclosed. This work has been the subject of several reviews.3 14 Only the work reported since the extensive Desimoni and Tacconi account5 of the Diels-Alder reaction of a,/3-unsatu-rated carbonyl compounds, 1-oxabutadienes bearing an oxygen atom at the diene terminus, has been detailed herein. The prior reviews should be consulted for an excellent discussion of the mechanism, scope, and application of the [4 + 2] cycloaddition reactions of a,/3-unsaturated carbonyl compounds as well as for extensive tabular compilations of the work through 1974.3-6... 

Methoxy-5,6-dihydro-2/i/-pyran-5-one (X) is subjected to the following transformations ... 

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