Cyclopropanes ring enlargements

There are relatively fewer examples of C3 - C5 cyclopropane ring enlargements which do not involve the vinylcyclopropane moiety. Most of them lead to thermal or... 

Benzo[b]oxepine carbojcylates 21 (structurally related to 18) have been synthesized from chromones 19 by sequential cyclopropanation/ring-enlargement reaction with diazoacetate in the presence of Tms-OTf/Cu(OTf)2 (Tms = trimethylsilyl) [9], for example ... 

Ye, S., Yu, Z.-X. (2011). TfOH-catalyzed tandem cyclopropane ring enlargement/C-C formation/etherification of alkynylcyclopropanes and 1,3-diketones to cyclobutane-fused dihydrofurans. Chemical Communications, 47, 794-796. 

Cyclopropanations by carbenes from chlorodiazirines were observed in several cases, e.g. with the r-butyl compound. Cyclopropanation and stabilization by ring enlargement and by elimination compete in chlorocyclobutyldiazirine photolysis. 

PFAU - PLATTNER Cyclopropane synthesis DIazoalkane insertion Into olelins wtth lormation of cyclopropanes or ring enlargement ol aromatics to cydoheptatnenes see also lormation ol pyrazoHnes (von Pechman). 

Acylation with imidazolides of a cyclopropane-CH activated by a phenylsulfonyl group proceeded under ring-enlargement to give a dihydrofuran system [103a]... 

The photoindueed 1,7-cycloaddition of carbon monoxide across the divinyl-cyclopropane derivative 32 yields the two cyclic dienyl ketones 34, via the ferracyclononadiene intermediate 33 [18]. (Scheme 11) cyclopentene rearrangement. The dienylcyclopropane 35 is capable of forming the complex 36, followed by ring enlargement to 37 [19]. 1,1-Dicyclopropylethylene 29 is also converted to the 1-cyclopropyl-1-cyclopentene 38. The additional functionality of vinylcyclopropanes is necessary to serve as a 7t-donor... 

The 3 + 2-cycloaddition of pyrroline A-oxide to 2-chloro-2-cyclopropylidene acetate and its spiropentane analogues (60) yields spiro [cyclopropane-1,5 -isoxazolidinejs (61) which undergo a novel cascade ring enlargement to yield indolizinones (62) in high yield (Scheme 21). ... 

A new selective thermal cascade ring-enlargement process of 4-chloro-substituted spiro[cyclopropane-l,5 -isoxazolidines], leading to a new method for the synthesis of the indolizine skeleton, has been reported (see Scheme 17). Apparently, the process is made possible by the presence of a chlorine substituent on the carbon a to the spirocyclopropane ring which facilitates a cyclopropyl-to-cyclobutyl ring enlargement mediated by a polar solvent. 

Electrophilic reagents undergo formal insertion into the cyclopropane bond and lead to ring-enlarged products. Reaction of 1 or 42 with dichlorocarbenes affords the corresponding dihalogenobenzocyclobutene (348). The same reaction has been realized with dicyclopropan hthalene (115). Reaction of 1 with SOj and SO3 affords the sultine 349 and the sultone 350, respectively. 4-Phenyl-1,2,4-tria-... 

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

The transformation of spiropentane to methylenecyclobutane can be regarded as a ring enlargement of a cyclopropane derivative (Section 7. A.3.3.), while the transformations of methylene-cyclobutanes can be classified as a rearrangement from one cyclobutane to another (Section 1.A.5.2.4.2.). The mechanistic aspects of these rearrangements were first studied in 1961 by Frey7y and Chesick.80... 

Oxaspiropentanes generally rearrange with inversion at the migrating terminus (see Section 3.2.2.2.). However, if a primary cation is involved, cyclopropylmethyl to cyclopropyl-methyl rearrangement with formation of a more stable secondary cation may precede the ring enlargement, and the stereochemistry of substituents in the cyclopropane part of the oxaspiropentane may be lost. This was found to be true for /ram-4,5-dimethyl-l-oxaspiropentane (5) where a considerable amount of m-2,3-dimethylcyclobutanone (m-6) was formed.47... 

Ring expansion reactions are much better known and sulfolene substrates have received considerable investigation. Cyclopropanation with dichlorocarbene affords 6,6-dichloro-3-thiabicyclo[3.1.0]hexane 3,3-dioxides which may readily be ring enlarged by base or acid treatment. Base treatment, especially with LDA, cleanly affords 2//-thiopyran dioxides hydrochloric acid does likewise, but less cleanly, while hydrobromic acid treatment causes a more complex set of reactions leading to thiopyranones containing bromine in which the sulfur has been reduced. The intermediacy of thiopyrylium oxides has been invoked to rationalize the observations (Scheme 27) (81JOC4502). 

Ring enlargement of chromone to benzoxepins has been reported (see also Section 7.07.5). The in situ generation of benzopyrylium triflate 335 by reaction of 334 with Me3SiOTf followed by addition of ethyldiazoacetate and catalytic copper(ll) triflate yielded cyclopropane 336 as a single diastereomer. Treatment of compound 336 with trifluoroacetic acid resulted in the formation of 2,3-benzoxepin 337 (Scheme 47). 

In several synthetic studies, cyclopropane derivatives were used as synthones or building elements for ring enlargement steps, e.g. reaction of enamines with cyclopropenone [65], synthesis of 2,3-dihydro-l,4-diazepine by thermal isomerization of 1,2-diamino-cyclopropanes [32] [66], and preparation of 3-amino-fulvenes from methylencyclopropenes with alkynamines [67]. 

---