- The result with 14b (Table I, entry iii) indicates that of the three acetals (benzylic, a-ethoxyethyl, and glycosidic) present in the molecule, only the last was removed during brominolysis.
By contrast, upon treatment of 14b with mild acid, the last was the only acetal to survive-as expected.
(4) Pursuant to the survival of the acid labile protecting groups, the result with the silyl ether 14c (entry iv) should be noted.
In general, the results with 14b -14g reveal that under the deglycosidation conditions, the benzylidene ring does not undergo the well-known H a n e ~s i a n -H u l l a r ' ~* ~~ reaction with NBS, a procedure widely used to cleave a benzylidene acetal while leaving a glycosidic center intact The formation of 15b -15g allows that pattern of chemoselectivity to be reversed.
( 5 ) In view of the reactions in entry i, survival of the 0-benzyl ethers in entry v ( 1 4 -15d) is not surprising. However, the oxidative conditions proved to be so mild that even the activated methoxybenzyl ether survived (14e -1%) to a substantial degree (entry vi).
(6) The results with the diacetate 14f (entry vii) were complicated by acetyl migration to give 16. (In this context, it should be noted that silyl migration15 was not observed with 1 5 . )
An obvious question relates to the formation of bromohydrins 1.70 (d, J(HH) = 6 Hz, 12 H, CHCH,), 3.90 (d of sept, J(PH) = 17.8 Hz, J(HH) = 6 Hz, 4 H, CH); mass spectrum m / e 388 (M'). Sa: "P NMR NMR (C6D6) 1.55 (d, J(HH) = 6 Hz, 12 H, CHCH,), 1.58 (s, 9 H, CCH,), (CDClJ +60.6 ppm; IR (C&) 2020 cm-I. Sb: -"P NMR (CDCI,) +68.8 ppm; IR (C&) 2040 Cm-'. &.I' 6b: "P NMR (C6D6) +65.7 ppm; IR (c&) 2050 cm-'. 7b: "P NMR (CDCIJ f68.5 ppm; IR (CJI6) 2020 c d . 14: "P NMR (CDCI,) +75.0, +57.9, J(PP) = 3.6 Hz; ')C NMR (CDC1,) 42.13 (dd, J(PC) = 26.4 and 4.5 Hz, CH,), 51.9 (s, CH,O), 61.7 (dd, J(PC) = 27.1 and 5.3 Hz, CHring), 144.2 (d, J(PC) = 151.6 Hz, C=N). 173.4 (s, C 4 ) ; IR (KBr) 1740 cm-' (CO), mass spectrum m / e 620 (M+). 15: "P CH,O), 120.8 (d, J(PC) = 27.2 Hz, =CH). 136.9 (t-like, J(PC) = 10.56 Hz, =C), 152.6 (dd, J(PC) = 148.7 and 3.0 Hz, C=N), 162.2 (s, CO); IR (CDCI,) 1730 (CO), 1590 (C=N) cm-I; mass spectrum m / e 618 (M+). (9) For a review see, for example: