Methyl 2.4- phenylboronate, oxidation

The crystalline phenylboronate derived by similar treatment of methyl a-L-fucopyranoside was shown to possess the 3,4-cyclic structure (25). This assignment is based on oxidation of compound 25 with methyl sulfoxide-acetic anhydride and the chromatographic identification of... 

The use of a boron atom as a covalent template for intramolecularizing the Diels-Alder reaction has been relatively little investigated [20-22]. However Narasaka et al. have reported the use of a boronate template to control regio- and stereoselectivity in an IMDA reaction between anthrone 50 and methyl 4-hydroxybut-2-enoate 3 [20a]. Formation of the boronate tether was achieved by simply heating an equimolar mixture of phenylboronic acid with the diene and dienophile at reflux in pyridine with azeotropic removal of water. After 5 h, a single cycloadduct 51 was obtained in 81% yield. The tether was readily removed oxidatively affording the corresponding diol 52 in excellent yield (Scheme 10-19). 

C-methyl and 2-C-methyl triacetates (480 and 481), with the latter distinctly prevailing. The same cycle of reactions applied to 479 gave the 5-thiopentose system (482) in low yield. Preparation of the 1,3-phenylboronate ester (483) followed by oxidation to a mixture of stereoisomeric sulfoxides and Pummerer rearrangement proved to be a better way the yield of both regioisomeric 2-C-methyl and 4-C-methyl compounds (484 and 485) was 20%. Compound 485... 

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