Ring oxygenation introduction

The reduction of 4-chloro-4-methyltetrahydropyran with triethylsilane requires more than a catalytic amount of aluminum chloride. No 4-methyltetrahydropyran is obtained after 20 hours at room temperature even when 0.75 equivalents of the catalyst is used, but a 92% yield is obtained after only 30 minutes when two equivalents of catalyst and three equivalents of triethylsilane are used.136 146 This is presumably a result of the ability of the Lewis acid to coordinate at the ring oxygen as well as at the chlorine. The introduction of alkyl groups at C2 appears to introduce enough steric hindrance near the ring oxygen to enable less than one equivalent of aluminum chloride to effect reduction, but also makes the products unstable to the reaction conditions so that the synthetic yields decline compared with the unsubstituted substrate.136... 

The possibility of this second type of interaction had not been considered previously. Its effect is a stabilization of equatorial chlorine. We must also remember that in some respect axial hydrogen plays the same part as axial chlorine (Figure 2). Consideration of all relevant interactions led to the formula given in Figure 4 (37). One important novelty is the introduction of the energy difference between the o and p-type lone pairs of the ring oxygen, a consequence of the fact that one pair stabilizes the conformation with axial chlorine and the other the conformation with equatorial chlorine. 

Similar lo the effect of ring oxygen atoms is the influence of a carbonyl group adjacent to the radical center 6. In allylation reactions of 2-oxocyclohexyl radicals, the formation of an axial product is slightly enhanced compared to the corresponding cyclohexyl radicals. This is in contrast lo the situation in cyclopentyl radicals, where the introduction of adjacent carbonyl groups leads to a lowering of cisjtrans selectivity in allylation reactions (see this section Cyclopentyl Radicals),... 

Monoterpenoids. It was mentioned earlier in this chapter that the semi-synthetic oxime, perillartine (1), is sweet and has some commercial use in Japan. In experiments designed to optimize the sensory attributes of the oxime sweeteners, it was found that the introduction of ether groups was advantageous, while hydroxyl groups and ring oxygen atoms tended to lower sweetness intensity and to destroy the sweet taste, respectively (16). Despite the discovery of cyclic derivatives of perillartine that are sweeter than this lead compound (20), the further development of this class of sweet substances is limited by poor water solubility and inappropriate hedonic qualities (20,21). 

Further modifications of the hydrophilic headgroup of such cyclic multiols have been realized. For example, the introduction of a pyranose ring in which the ring-oxygen atom is replaced by nitrogen offering for the first time the possibility of substitution at this particular position of the cyclic head-group of an amphiphile (see Fig. 16) [100],... 

Ethers have been prepared by the thermal lead tetraaeetate method in 60-71% ° yield. Introduction of an axial 3a-acetoxy function into 5a-H steroids, however, seems to change the conformation of ring A in such a way that the distance of the 2/ -oxygen from the angular methyl group is considerably increased. Consequently the 2/5,19-ether is formed in only 0.7% to 24% yield. " 02... 

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