scope for this as few can be produced conveniently and nearly all are unstable (see: carbene dimerization). An exception are dihalocarbenes such as dichlorocarbene or difluorocarbene, which are reasonably stable and will react to form geminal dihalo-cyclopropanes. These compounds can then be used to form allenes via the Skattebøl rearrangement. The Buchner ring expansion reaction also involves the formation of a stabilised carbene. Cyclopropanation is also stereospecific as the addition of carbene and carbenoids to alkenes is a form of a cheletropic reaction, with the addition taking place in a syn manner. For example, dibromocarbene and cis-2-butene yield cis-2,3-dimethyl-1,1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane. From alkenes using ylides Cyclopropanes can be generated using a sulphur ylide in the Johnson–Corey–Chaykovsky reaction, however this process is largely limited to use on electron-poor olefines, particularly α,β-unsaturated carbonyl compounds. Intramolecular cyclisation Cyclopropanes can be obtained by a variety of intramolecular cyclisation reactions. A simple method is to use primary haloalkanes bearing appropriately placed electron withdrawing groups. Treatment with a strong base will generate a carbanion which will cyclise in a 3-exo-trig manner, with displacement of the halide. Examples include the formation of cyclopropyl cyanide and cyclopropylacetylene This mechanism also forms the basis of the Favorskii rearrangement. A related process is the cyclisation of 1,3-dibromopropane via a Wurtz coupling. This was used for the first synthesis of cyclopropane by August Freund in 1881. Originally this reaction was performed using sodium, however the yield can be improved by exchanging this for zinc. BrCH2CH2CH2Br + 2 Na → (CH2)3 + 2 NaBr Other approaches The Kulinkovich reaction form cyclopropanols via a reaction between esters and Grignard reagents in presence of a titanium alkoxide. The Bingel reaction is a specialised cyclopropanation reaction used to functionalise a fullerene. In the di-π-methane rearrangement, photochemical stimulation causes 1,4-dienes to rearrange to form vinylcyclopropanes. These can then undergo vinylcyclopropane rearrangements Cyclopropane-fatty-acyl-phospholipid synthase performs cyclopropanation in biological systems Biosynthesis Although cyclopropanes are relatively rare in biochemistry, many cyclopropanation pathways have been identified in nature. The most common pathways involve ring closure reactions of carbocations in terpenoids. Cyclopropane fatty acids are derived from the attack of S-adenosylmethionine (SAM) on unsaturated fatty acids. The precursor to the hormone ethylene, 1-aminocyclopropane-1-carboxylic acid, is derived directly from SMM via intramolecular nucleophilic displacement of the SMe2 group subsequent to condensation with pyridoxal phosphate. Direct carbene transfer from diazoesters to olefins has also been achieved