Rearrangement backbone

See Terpenoids and Steroids , ed. K. H. Overton (Specialist Periodical Reports), The Chemical 

109 j Prange, C. Pascard, G. Sozzi, and M. Fetizon, Tetrahedron Letters, 1977, 245. 

The 3-oxo-l,5-cyclocholestane (158) rearranges with BF3 and with HCOjH to give the enone (159) electrophilic attack at the carbonyl oxygen is followed by cleavage of the 1,5-bond to produce an intermediate C-5 carbocation. A minor product (160) in the BFs-catalysed reaction is derived from cleavage of the 1,10-bond. Addition of methanol or ethanol across the 1,5-bond is catalysed by FeCU.bHzO. 

Chlorination of the ITa-hydroxy-16-methylenepregnan-20-one (161) gave a small amount of the unusual oxetan (162) in addition to the expected 16a, 17a- 

15a-epoxycholest-7-ene (170) occurred with LiAlH4 or LiEtjBH to give the 15a-hydroxycholest-8(14)-ene (171) attack of hydride ion at C-7 is established by 

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Since none of the lattice models is now clearly superior, the choice for interpretation of spin relaxation in polymers is arbitrary. Familiarity leads us to select the Jones and Stockmayer model so we will now consider application of this model to several well studied polymer systems in order to compare dynamics from polymer to polymer. Also the equations required to consider anisotropic Internal rotation of substituent groups and overall molecular tumbling as independent motions in addition to backbone rearrangements caused by the three-bond jump are available for the Jones and Stockmayer model (13). 

The relationship between internal rotation of substituents and backbone rearrangements can be considered from the interpretation. The time scales of anisotropic internal rotation and backbone rearrangements are well separated in M2PPO. In addition, the concentration and temperature dependences of these two quantities are quite different leading us to conclude that the motions are independent. In PIB and PS, Internal rotation and backbone... 

Relationship between Backbone Rearrangement and Internal Rotation... 

The other class of motion only now being introduced into interpretive models is oscillatory motion. Anisotropic oscillatory motions of substituent groups have been considered by Chachaty (12) but not in conjunction with a lattice description of backbone motion. No attempt to develop a model based on oscillatory backbone rearrangements is known to these authors, and this avenue may be very important for the interpretation of concentrated solutions, rubbery or amorphous solids, and especially glassy polymers... 

Using 5(3,6(3-epoxycholestan-3(3-yl acetate as substrate, other solvents were also screened in this reaction and it was observed that using MeN02, a solvent with higher dielectric constant, the preferential formation of the backbone rearranged derivative occurred, but the Westphalen-type compound was also formed (Scheme 31). 

Scheme 32 Bi(0Tf)3-xH20-catalyzed Westphalen and backbone rearrangement of 5P,6P-...

The application of Westphalen and backbone rearrangement reaction conditions (MeN02, Bi(0Tf)3xH20 catalyst) [118] to 16a,17a-epoxy-20-oxosteroids has also been studied [124]. At room temperature and 0.05 eq. of catalyst, low conversion of 16a,17a-epoxy-20-oxopregn-5-en-3p-yl acetate was observed. However, on increasing the temperature to 50 °C, 16a- and 16p-hydroxy-17p-methyl-A13-18-nor pregnane derivatives were formed in near similar amounts from 16a,17a-epoxy-20-oxosteroids (Scheme 33). 

Scheme 35 Bi(0Tf)3-xH20-catalyzed backbone rearrangement of 5p,6P 16a,17a-diepoxy-20-oxopregnan-3P-yl acetate...

The transformation of the lanostane keto-epoxide (72) into the cucurbitacin derivative (73) has been achieved with boron trifluoride in acetic anhydride. The presence of the acetic anhydride appears to favour the backbone rearrangement. The structure of (73) was confirmed by correlation with the oxidation product (74) of deoxybryogenin acetate. Two new cucurbitacin glycosides, arvenins 1 (75) and II (76) from Anagallis arvensis, have been reported. ... 

Methyl trisnorshionanoate (126) has been synthesized from friedelan-19a-ol (127) via the ring-contracted intermediate (128) and the trisnor-ketone (129). Irradiation of (129) in methanol afforded the desired ester (126). An investigation of the effect of solvent on the backbone rearrangement of 3/8,4/3-epoxyshionane (130) by boron trifluoride etherate has shown that nucleophilic solvents tend to interrupt the rearrangement at an early stage (see Vol. 6, p. 130). 

The diterpene derivative 1 was treated with BF3-OEt2 and AC2O in an attempt to induce a "pinacol-type rearrangement leading to a ring B-homo derivative. No such reaction was observed. Instead, the only product which could be isolated (33%) was shown to be 2, formed by "a profound backbone rearrangement" of 1. 

From the FTDOCK simulations described above, it is obvious that small localized backbone rearrangements in the protein-protein interface can... 

By analogy with what has gone before, you might now expect a tame hydration or reduction of this cation. Nothing of the sort A rearrangement occurs in which five consecutive 1,2-shifts are followed by an elimination. Since this reaction organizes the backbone of the steroids, it is often called the steroid backbone rearrangement. 

There are quite a lot of differences between the chemical and the biochemical versions so far—the chemical ones are less complex and less sophisticated but more versatile. The reactions arc just cyclizations without the backbone rearrangements. The most important points of difference are ... 

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