De novo synthesis of carbohydrates and related natural products, 34. Synthesis of N‐acetyl‐β‐D‐neuraminic acid derivatives via inverse‐type hetero‐Diels‐Alder reaction

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Abstract
Inverse‐type hetero‐Diels‐Alder reaction‐based diastereoselective synthesis of 3‐deoxy‐2‐glyculosonates is performed with the help of chiral carbon substituents in the 2‐position of the 1‐oxa‐1,3‐diene required as heterodiene. This is demonstrated for the synthesis of the selectively O‐protected and the O‐unprotected N‐acetylneuraminic acid derivatives 21 and 22, respectively. Thus, the heterodiene 10, obtained from D‐mannose in high overall yield, affords with methyl α‐methoxy‐acrylate as heterodienophile preferentially the stereoisomer 11 with β‐D‐ribo configuration. Phenylthio group removal, diastereospecific 5‐hydroxy and 6‐hydrogen transfer, and then stereocontrolled introduction of the acetamino group into the 5‐position furnished compound 21 with the required D‐glycero‐D‐galacto configuration. Structural proof comes from the transformation into the known N‐acetylneuraminic acid derivative 22.