Stereoselectivity ring contraction

In a similar way 1,6 anhydro 2-ketopyranoses63 of the xylo, ribo and psico series underwent stereoselective ring contraction by decarbonylation (Scheme 33). 

The reduction under free radical conditions of cyclic carbamate 213 effected a stereoselective ring contraction to provide oxazolidinone 214 <02JOC1972>. 

The 0-benzyl derivative of the glycal 126 undergoes stereoselective ring contractions on treatment with thallium(III) nitrate [110] (O Scheme 30). 

Stoltz, B. M., Wood, J. L. The stereoselective ring contraction of a pyranosylated indolecarbazole. A biosynthetic link between K252a and staurosporine Tetrahedron Lett. 1996, 37, 3929-3930. 

Stannylcyclohexanones undergo a stereoselective ring contraction when treated with TMSOTf at low temperature. When other Lewis acids were employed, a mixture of ring-contracted and protiodestannylated products was obtained (eq 21). ... 

Danishefsky and coworkers has approached Terreulactone A, with their efforts outtined in Scheme 12.58 [107]. The enol intermediate 242 was readily obtained from the Wieland-Miescher ketone 241 by chemo- and stereoselective reduction, protection of die resultant alcohol, dimethy-lation, followed by a two-step oxidation sequence. Then the enol intermediate 242 was subjected to a stereoselective ring contraction condition to afford 243. Lactonization of 243 followed by appropriate functional group management provided the A, B, C rings of terreulactone A 245, which they hoped to convert to 240 and expected to couple 240 with a suitable E,F progenitor 239 by the [3+3] cycloaddition strategy. Their completion of the total synthesis of terreulactone A 238 is under way. 

Marshall has reported an innovative application of the Wittig rearrangement in the synthesis of the 14-membered cembrenoid carbocycles (Scheme 16.21) [104], The stereoselectivity of this process is coupled with the conformational preferences of the macrocyclic ether. The rearrangement reaction of the 17-membered macrocycle 187 provided a 90% yield of the 14-membered alcohols, with 188 as the major diastereomer (dr=81 7 6 7). This represents a remarkable example of macrocyclic stereocontrol (see also Chapter 1) to effect a stereoselective ring-contraction by use of a Wittig rearrangement. 

Cyclopentene-l-carboxaldehydes are obtained from cyclohexene precursors by the sequence cyclohexene - cyclohexane-1,2-diol -> open-chain dialdehyde - cyclopentane aldol. The main advantage of this ring contraction procedure is, that the regio-and stereoselectivity of the Diels-Alder synthesis of cyclohexene derivatives can be transferred to cyclopentane synthesis (G. Stork, 1953 G. BUchi, 1968). 

Several ring contraction routes to /3-lactams have been developed. One of the most important is the photochemical Wolff rearrangement of 3-diazopyrrolidine-2,4-diones (178), which provides a general route to 3-carboxyazetidin-2-ones (179). Lack of stereoselectivity is a problem, but facile epimerization is possible because of the electron withdrawing 3-substituent (78T1731). 

In analogy to Edstrom s experiments, the nine- and ten- membered ring lactams underwent regio and stereoselective transannular ring contractions to give the corresponding indolizidinones and quinolizidinones, respectively (vide supra Scheme 56) (Table 21) [61]. 

The base-catalysed ring contraction of 1,3-dioxepanes offers an attractive route to 4-formyl tetrahydropyrans (Scheme 14) , whilst fused exo-cyclic dienes 27 result from the radical cyclisation of alkenyl iodides 26 (Scheme 15) <00OL2011>. Intramolecular radical addition to vinylogous sulfonates is highly stereoselective, leading to the ci s-2,6-disubstituted tetrahydropyran (Scheme 16) . 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

The bromoacetal 95 undergoes ring contraction to the relatively unstable aldehyde 96a in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or triethyamine (Scheme 9) <1995JME1922>. This aldehyde 96a has been obtained in a one-pot process from anhydrodihydroartemisinin 25 in excellent yield. The stereoselectivity of the reaction with iodine is opposite to that with bromine, and allows the preparation of the isomeric aldehyde 96b, with conservation of the configuration of the starting artemisinin at C-9 <2001TL2125>. 

The diverse chemistry of carbenes is beyond the scope of this account, but a few typical reactions are shown here to illustrate the usefulness of the photochemical generation of these reactive species. A carbene can insert into a C—H bond, and this finds application in the reaction of an a-diazoamide to produce a P-lactam (5.29). Carbenes derived from o-diazoketones can rearrange to ketenes, and thus a route is opened up to ring-contraction for making more highly strained systems <5.301. Carbenes also react with alkenes, often by cycloaddition to yield cyclopropanes in a process that can be very efficient (5.31) and highly stereoselective (5.321. 

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