Ring-expansion approach

Additionally, the authors chose 3-chloropropionyl chloride as the immobilized building block in order to carry out a ring-expansion approach, which led to the generation of a 14-member library of thioxotetrahydropyrimidinones [85, 86], The initially prepared polymer-bound chloropropionyl ester was efficiently transformed into the corresponding diamines by transamination utilizing several primary amines. These diamine intermediates could also be obtained by treatment of the pure polymeric support with acryloyl chloride and subsequent addition of the appropriate amines (Scheme 7.74). 

Asymmetric IRDARs of optically active furfuryl fumarates 91 were investigated under thermal and high-pressure conditions. The diastereoselectivities observed increased with the size of the tether substituents, achieving up to 86% in the case of R = f-Bu, though in the case of R = neo-Pen only 38% de was obtained. It is concluded the diastereoselectivity observed was thermodynamically controlled (Scheme 27) [65]. An IRDA ring-expansion approach toward taxinine (a carbocylic compound) [66] utilizing both Lewis acids and high pressure has been reported [67]. 

A concise synthesis of the novel pyrrolo[l,2-a]benzodiazepine system 92, using the metallocarben-oid/spiro-[6,5]-ammoniumylide/Stevens[l,2]-shift with ring-expansion approach, was reported. The overall cascade process was stereospecific <07T12232>. 

In 1996, the same group 87) reported further work on the synthesis of indole analogues of the cephalotaxine ring system (Scheme 52). The key intermediate in this ring-expansion approach, bromoiminium ion 300, was prepared in three steps from tryptamine and the chloro diester 298 via enamine 299. On treatment with several bases, the iminium ion 300 underwent rearrangement, presumably via the intermediate alkoxide 301, to give the azepinone derivative 302, which was reduced with sodium borohydride to a mixture of isomeric alcohols 303. The alcohols 303 underwent rapid intramolecular cyclization when treated with a 95% solution of sulfuric acid to yield pentacyclic ketone 304. 

For the ring expansion approach to 41, only symmetrical 1,2-diaryl substitution of the five-membered ring of the oxadiazolium salt is possible, since unsymmetrical ring substitution of the starting furazans would yield a mixture of... 

Scheme 6 A Beckmann ring expansion approach to tetrahydropyrido[3,2-b]azepin-...

Retrosynthetic analysis The development of a ring expansion approach to the pyranosylated... 

Thornton, P.D. and BumeU, D.J. (2006) A Pauson-Khand and ring-expansion approach to the aquariane ring system. Org. Lett., 8, 3195-3198. 

There are several mechanistically related ring expansion reactions of cyclopropanones which lead to /3-lactams. The conversion of cyclopropanone to /3-lactam (174) via the cyclopropanolamine (173) (75JOC1505) is just one modification, but it illustrates the strategy of this type of approach (73TL4855, 69JA2375) which has been applied to the synthesis of 3-amino-nocardicinic acid (81JOC2999). 

Further work on this approach to deoxy sugars is in progress, particularly to determine the directive influences which operate in the ring expansion. 

Azepines are much less common than the 1H- and 3//-isomers, and one of the few general synthetic approaches to this system is by base-catalyzed ring expansion of 4-(chloromethyl)-l, 4-dihydropyridines. e. g. I.29 Curiously, the ring expansion is also effected by potassium cyanate in refluxing ethanol. 

For the synthesis of the complex natural product, the terminus six-membered ketone 55 had to be transformed into an oxepane ring. For this necessary transformation, the authors were attracted by the single-carbon homologation of a pyr-anone (a sort of ring-expansion) because, in prindple, it could be used in an iterative sense at any stage of the 6-endo cydization in their poly-TH P-based synthetic approach for the synthesis of trans-fused 6,7,6 (THP-oxepane-THP) and 6,7,7 (THP-oxepane-oxepane) ring systems [28]. Treatment of ketone 55 with TMSCHN2... 

ROCM also offers the possibility of a ring expansion. Tadano and co-workers used this approach successfully as a key step in the total synthesis of (H-)-mycoep-oxydiene, though only in moderate yield [165] (Scheme 3.17). 

Trost s approach to gibberellins applied the ring expansion of the epoxides (165)... 

The reaction of allenylsilanes with a,/8-unsaturated acylsilanes presents a new [3 + 3]-cycloaddition approach to a six-membered carbocycle [189]. Lewis acid-promoted ring expansion of the [3 + 2]-annulation product 260 is followed by a second cationic 1,2-silyl migration to produce the cyclohexenone 261 after desilylation. 

A sequential process of carbopalladation and ring expansion of allenylcyclobuta-nols provides five-membered rings (Scheme 16.19) [24], The intramolecular version of this approach offers a facile synthetic method for 5,7- and 5,8-fused ring frameworks (Scheme 16.20) [24]. 

Poly(3-hydroxyalkanoate)s have potential for application to engineering plastics endowed with biodegradable nature. One of the synthetic approaches to the polyesters is the ring-opening polymerization of -substituted /3-lactones which can be effectively produced by ring-expansion carbonylation of epox-... 

Scheme 6.9 shows the radical clock methodology approach used for obtaining the rate constant of the ring expansion (k e)- Radical 37 was obtained by the reaction of the corresponding phenylseleno derivative with (TMS)3SiH. A relative rate constant of = 20.2 was obtained at 80 °C under first-... 

Scheme 6.9 Radical clock methodology approach for measuring the rate constant of ring expansion (cf. Section 3.1.1)...

A flexible approach to the synthesis of substituted benzazepines 403 was devised (equation 166). As before, the seven-membered ring was made by the use of a ring-expansion reaction and exclusive migration of the aromatic ring was observed. 

Fig. 6. Septanosides 120-124 prepared by Jayaraman via the ring expansion/oxepine approach.

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