Three-Membered-Ring Formation

This is a very strained ketone but three-membered ring formation is kineticalJy favourable and the hindered base offers no cleavage reactions. Notice that the rings are added in order of size. 

In the reaction of diazofluorene with PTAD the dipolar intermediate (87) is isolable.136 Collapse to a five-membered ring is prevented by the steric constraints of the cyclic system, and although three-membered ring formation is possible, dimerization is the only process observed on heating. However, the dipole reacts with a variety of dipolarophiles (a=b, PhN=CO, Me02CC CC02Me, etc.) to give the expected adducts as shown in Scheme 11. 

Another important line of investigation concerned the carbonyl insertion reaction, which was best defined in manganese chemistry (75, 16) and extended to acylcobalt tetracarbonyls by Heck and Breslow. The insertion may be through three-membered ring formation or by nucleophilic attack of an alkyl group on a coordinated CO group. 

The former Chapter Reaction of Ylides with Saturated or a,P-Unsaturated Carhonyl Compounds got rid of its three-membered ring formations and the rest strictly remodelled to lumish the new Chapter 11 Reaction of Phosphorus- or SuHur-stabilized CNucleophiles with Carhonyl Compounds Addition-induced Condensations . 

Reactions of S Ylides with Saturated Carbonyl Compounds or with Michael Acceptors Three-Membered Ring Formation... 

Stereoselectivity and Regioselectivity of Three-Membered Ring Formation from S Ylides... 

Three-membered ring formation is fast—the product is strained so AH is large but this is offset by the reacting atoms being as close as they can get in a freely rotating chain... 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

Path NuL, Nu-L Additions (Three-membered ring formation)... 

The Nu-L class of reagents can be recognized by a nucleophilic center to which a leaving group is directly bonded. This unusual arrangement in which the same atom can serve both as a sink and a sonrce is characteristic of the Nu-L class, and thus their reactions are reasonably similar, most often three-membered ring formation or addition followed by rearrangement. Below are several examples of the Nu-L class. 

The general pattern of diaziridine synthesis is probably also followed by three-membered ring formation from perfluorinated aminals " as well as by a diaziridine synthesis from hydrazoic acid." In the course of the difluorodiazirine synthesis of Mitsch," perfluoromethylenediamine 96 is defluorinated by ferrocene. 

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