Allenes pyrans, dihydro

Saalfrank, Hoffmann and co-workers performed a number of reactions with tetra-alkoxyallenes such as 196 (Scheme 8.47) [1, 41, 105, 114—116] and demonstrated that this class of donor-substituted allenes can serve as a 1,3-dianion equivalent of malonic acid. Treatment of 196 with cyclopropyldicarboxylic acid dichloride 197 produces 2,4-dioxo-3,4-dihydro-2H-pyran 198 through release of two molecules of ethyl chloride [115]. Similarily, the reaction of this allene 196 with oxalyl chloride gives 3-chloromalonic acid anhydride derivative 199. This intermediate is a reactive dieno-phile which accepts 2,3-dimethyl-l,3-butadiene in a subsequent [4+2] cycloaddition to afford cycloadduct 200 in good yield [116]. 

Cycloaddition reactions of electron-rich allenes with some heterodienes take place at the C1-C2 bond of the allene to yield heterocycles, a,Unsaturated carbonyl compounds 185 react with the internal C=C bond of ethoxyallene to afford dihydro-pyrans in moderate yields [150]. 

Enol ethers and enol acetates of cyclic 1,3-dicarbonyl compounds also afford mixtures of regioisomers on irradiation in the presence of allene, the preferential orientation of addition for enol ethers being mainly head-to-tail.9-11 Endocyclic enol ethers on the other hand, e.g. 2.3-dihydro-4//-pyran-4-ones 4, add regioselectively to allene with exclusive formation of head-to-head adducts.11... 

The conversion of /3-allenic alcohols into 5,6-dihydro-2F/-pyrans is catalyzed by silver ions and takes place under mild conditions (79S743). 

The ether-tethered allenyne 4 is converted into a cross-conjugated triene, the 5,6-dihydro-2tf-pyran 5, in a Rh-catalysed allenic Alder ene reaction <02JA15186> and allylic halides undergo a Pd-catalysed coupling reaction with 3,4-allen-l-ols to give 5,6-dihydro-2//-pyrans (Scheme 2) <02JOC6104>. 

Representative procedure for the gold-catalyzed cyclization of allenic malonate esters. Methyl 3-(cyclohex-2-enyl)-3,6-dihydro-6,6-dimethyl-2-oxo-2H-pyran-3-carboxylat ... 

A reductive homocondensation of ( )-benzylidenepyruvate esters promoted by P(NMe2)3 affords polyfunctionalized 3,4-dihydro-2f/-pyrans (130L1926). Enantiopure 5,6-dihydro-2f/-pyrans are accessible from a chemo- and regio-controlled palladium(II)-catalyzed domino reaction of P,Y-allenediols with a-allenic acetates (Scheme 3) (13CEJ14233). 

The 2,3-dihydro-4-/7-pyran-4-ones 189 react with allene stereospecifically to give two isomeric cycloadducts 190 and 191. ... 

Less coimriDn are cyclizations of -allenic heterosubstituted systems to form the corresponding six-membered heterocyclesT A silver-mediated, high yielding cycloetherfication was used as one of the key steps for the constraction of the dihydro-2ff-pyran within the tricyclic framework of eunicins (eq 15)T ... 

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