Hemiacetal Ring Opening

Caracurine V can be reduced catalytically over rhodium (93) to give two stereoisomeric bases, both of which are tetrahydro derivatives formed by saturation of the 19, 20, and 19, 20 double bonds. These bases can be iV-methylated, whereupon the hemiacetal rings open to afford the isomeric tetrahydrotoxiferine and isotetrahydrotoxiferine (LXXII R = OH with 19,20 and 19, 20 double bonds saturated). 

Vassilikogiannakis and coworkers described a simple sequential process for the biomimetic synthesis of litseaverticillol B (4-159) which includes a cycloaddition of 4-158 and singlet oxygen to give 4-160, followed by ring opening to afford the hydro-genperoxide 4-161 (Scheme 4.34) [55]. Reduction of 4-161 led to the hemiacetal 4-162, which underwent an aldol reaction to afford 4-159. 

A rather complex microwave-assisted ring-opening of chiral difluorinated epoxy-cyclooctenones has been studied by Percy and coworkers (Scheme 6.131) [265]. The epoxide resisted conventional hydrolysis, but reacted smoothly in basic aqueous media (ammonia or N-methylimidazole) under microwave irradiation at 100 °C for 10 min to afford unique hemiacetals and hemiaminals in good yields. Other nitrogen nucleophiles, such as sodium azide or imidazole, failed to trigger the reaction. The reaction with sodium hydroxide led to much poorer conversion of the starting material. 

In animals, ascorbic acid is synthesized in the liver from o-glucose, by a pathway that initially involves specific enzymic oxidation of the primary alcohol function, giving o-glucuronic acid (see Section 12.8). This is followed by reduction to L-gulonic acid, which is effectively reduction of the carbonyl function in the ring-opened hemiacetal. 

Kinetic studies of the ring opening of (71JA2733) and the cyclization to (74CR(C)(279)697) 2,4,6-triphenylpyrylium salts have been carried out. A bifunctional catalytic action of acetic acid on both the enolization and ring closure of the hemiacetal has been postulated in the latter study. 

Sodium borohydride and lithium aluminum hydride afford the hemiacetal (16), but aryllithium reagents effect arylation and concomitant ring opening to hydroxyketones (17 Scheme 3) (79ZOR2405). 

It is reasonable to assume that reducing sugars which exist in the pyranose or furanose form must revert to their open-chain aldehydo or keto forms before any significant C—H ionization occurs. This reversion is brought about by ionization of the acidic hemiacetal hydroxyl group, followed by ring opening. Subsequent carbanion formation at the carbon... 

C Hemiacetal rings are stable and do not easily open to form straight chain aldehydes. 

The cation formed by ring opening is captured by a water molecule to yield the hemiacetal product. 

When we first investigated processed foods for the occurence of RP s there was no direct method for the determination of their conformation. An RP can exist in the open keto-forra or hemiacetal ring structures derived from it (Figure 1)... 

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