Hemiacetals bridged

Tautomerism involving hemiacetal bridges has been explored by C n.m.r. for aldosterone (16), 18-hydroxy-11-deoxycorticosterone (19), and 18-hydroxy-progesterone (18)." Aldosterone and its 21-acetate (17) each showed 32 lines in their broad-band proton-decoupled spectra, indicating the presence of both the... 

Moreover, compounds of type 7-127, which were obtained from 7-125 by reaction with Pb(OAc)4, can undergo a further domino process when treated with potassium carbonate in a mixture of water and methanol [57]. This includes saponification of the acetate moieties in 7-127 to provide 7-129 via the unstable cyclic hemiacetal 7-128 (Scheme 7.35). Retro-Claisen reaction and ring closure with the proposed intermediates 7-130 and 7-131 led to the bridged ring-system 7-132 as a mixture of di-astereomers with preference of the (3-isomer. 

Some unusual nucleophile functional groups that have been utilized in cyclofunctionalizations to form bridged ring systems include cyclic hemiacetals (equation 18)73 and epoxides (equation 19).87 The cyclizations in equation (20) involve a cyclic enol ether as the reactive ir-system and have been used in syn-... 

The oxidative cyclization of an ortho-hydroxy-methoxy-substituted aromatic system giving a methylenedioxy group is also known to involve a cytochrome P-450-dependent mono-oxygenase. This enzyme hydroxylates the methyl to yield a formaldehyde hemiacetal intermediate, which can cyclize to the methylenedioxy bridge (the acetal of formaldehyde) by an ionic mechanism (Figure 2.21). 

Reaction of tetrahydro-4/7-thiopyranone 795 with meso-794 gave the bridged oxathiocins 775 in 68% yield and 92% ee, respectively. Oxazocine 775 existed exclusively in the hemiacetal form, suggesting that the stereocenter originating from C-6 in 794 could be set under thermodynamic control. Reaction of 795 with the readly available 3.5 1 mixture of ( )-794 and meso-794, respectively, produced 775 in the same yield and ee. 

As final examples of the versatility of the Ritter reaction, bridged hemiacetals react by means of their acyclic form in solution to afford amido ketones in good yield (equation 29), while halohydrins give a-amido halides, which are readily cyclized to oxazoline derivatives in an overall stereospecific process. ... 

Compounds showed weak activity in both assays, due to their hemiacetal structure. In general, all compounds showed potent antifeedant and insecticidal activity at concentrations of 16.0-63.7 pg/cm2. These compounds contained the following partial structures an a,P-unsaturated carbonyl in ring A, vicinal hydroxy groups at Cn and Ci2, a methyleneoxy bridge between Cg and C13 and a side chain atCis [111]. 

It appears that the energy of the absorbed photon, after absorption by the aglycon portion, must be transferred intramolecularly to the hemiacetal oxygen bridge (which is the reactive center). Although the effect is small, the efficiency of this transfer is greater for benzyl than for phenethyl, but phenyl n-glucosides are the least efficient of those studied. 

Dialdehydes obtained from methyl aldohexopyranosides and aldopentofu-ranosides (for instance, 44, which is formed from a-o anomers) may assume a multitude of structures, because, in addition to acyclic and hemialdal forms, they can exist as internal hemiacetals. A crystalline, dimeric form of the latter type (69) has been characterized. However, 44 affords different derivatives that represent various types of structures, and that can, therefore, best be described as a complex, equilibrium mixture of several forms.Condensation of the dialdehyde derived from uridine with benzoylhydrazine effected replacement of the bridging oxygen atom of the hemialdal unit to afford an A-benzamidomorpholine derivative. ... 

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