Hetero Diels-Alder Additions of 1-Oxa-1,3-dienes

The [4 + 2] cycloaddition of a,p-unsaturated aldehydes and ketones (1-oxa-1,3-dienes) to enol ethers (Diels-Alder addition with inverse electron demand) has been an attractive route for the synthesis of 3,4-dihydro-2 -pyrans [143-146], which can be converted into deoxy- and dideoxypyranosides [147,148]. 

The Diels-Alder addition of enantiomerically pure oxadienes to (Z)-2-ethoxyvinyl acetate are highly diastereoselective and have allowed Tietze and coworkers to prepare derivatives of 4-deoxy-D-/yv(9-hexose [146]. 

SCHEME 13.81 Dujardin s synthesis of 2-deoxy-L-araZ /no-hexose. 

The first catalytic, highly enantio selective hetero Diels-Alder reactions of thiabutadienes with an acyloxazolidinone dienophile using homochiral copper and nickel triflate and perchlorate bis(oxazoline) and bis(imine)complex catalysts to generate dihydrothiopyrans were reported by Saito et al. [155]. 

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The enantiopure 1-oxa-1,3-diene 278 is prepared by acylation of benzyl vinyl ether with oxalyl chloride. This generates acyl chloride 276, which acylates the lithium salt of 2-oxazolidinone 277. In the presence of Me2AlCl, diene 278 adds to (Z)-l-acetoxy-2-ethoxyethene giving mostly adduct 279, whereas, when using Me3SiOTf as promoter of the hetero Diels-Alder addition, diastereomer 280 is the major adduct (Scheme 13.79). Adducts 279 and 280 have been converted into ethyl P-d-mannopyranoside and ethyl-P-L-mannopyranoside, respectively [149]. 

In addition to the mentioned oxa-, aza- and thia- 1,3-butadienes and the hitherto discussed dienophiles, a multitude of polyhetero dienes and -dienophiles have been employed in recent hetero Diels-Alder reactions. The array of heteroatoms amenable for such cycloadditions is by no means restricted to oxygen, nitrogen and sulfur since there exist numerous reactions involving less common heteroatoms such as phosphorus, silicon or selenium. 

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