Pyran-2-ones, cycloaddition/allylic

Scheme 2.65 Tandem [2 + 2] cycloaddition/allylic rearrangement to pyran-2-ones [122, 123).

Dihydro-2H-pyran-2-ones (e. g., 4-195) are valuable intermediates in the synthesis of several natural products [67]. Hattori, Miyano and coworkers [68] have recently shown that these compounds can be easily obtained in high yield by a Pd2+-catalyzed [2+2] cycloaddition of ct, 3-unsaturated aldehydes 4-192 with ketene 4-193, followed by an allylic rearrangement of the intermediate 4-194 (Scheme 4.42). In this reaction the Pd2+-compound acts as a mild Lewis acid. a,(3-unsaturated ketones can also be used, but the yields are below 20%. 

Fused 2//-pyran-2-ones are formed in excellent yields and under mild conditions through a Ni-catalysed [2+2+2] cycloaddition of diynes and C02 <02JA15188>. Oxidative demetalation of 0i3-allyl)Mo complexes of pyran with pyridinium dichromate (PDC) introduces a carbonyl function at the allylic terminus offering access to dihydropyranones of high enantiopurity <02JOC5773>. 

The 4-vinyloxetan-2-ones generated in a Pd-catalysed [2+2] cycloaddition between ketene and a,P-unsaturated carbonyl compounds undergo a spontaneous allylic rearrangment. A zwitterionic intermediate is proposed that cyclises to a 3,6-dihydro-2//-pyran-2-one, but which may alternatively decarboxylate to a diene <02T5215>. 

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