Cyclopropyl ethers, synthesis

Oxaspiropentanes are classified as a unique class of cyclopropyl ethers. Two reaction courses are recognized as being associated with the reaction in the presence of lithium bases. Treatment of oxaspiropentanes with butyllithium leads to S 2-type epoxy ring opening, giving butyl-connected cyclopropanols. On the other hand, with lithium amide as base there is deprotonation at the epoxy carbon followed by epoxide ring opening. The latter reaction offers a useful synthesis of 1-vinylcyclopropanols. ... 

This reaction has been carried out on cyclopropanols these substances, however, are less available (2, 118-119) than trimethylsilyl cyclopropyl ethers and are readily decomposed by the hydrogen bromide formed on bromination. In the new synthesis, the by-product is trimethylbromosilane. 

A variety of different routes to cyclobutane systems involving non-photochemical means has also been devised. Acid catalyzed rearrangement of fused cyclopropyl ether (61) by Wenkert et al. (97, 98) afforded the bicyclic dione (62), which through a thioketal-desulfurization process and treatment with hydroxylamine yielded oxime (58) (Scheme 12). The shortest synthesis of grandisol is that reported by Billups et al. (Scheme 13) (99). Dimerization of isoprene in the presence of a zero-valent bis-cyclooctadienyl-nickel-phosphite complex gave the cis-cyclobutane diolefin (65), which could be separated in 12—15% yield from the complex product mixture by low-temperature distillation. Selective hydroboration and oxidation afforded grandisol. 

Intermolecular cyclopropanation reactions with ethyl diazoacetate have been employed for the construction of the cyclopropane-containing amino acid 7 (equation 25) Thus, rhodium(II) acetate catalysed decomposition of ethyl diazoacetate in the presence of d-cbz-vinylglycine methyl ester 5 afforded cyclopropyl ester 6 in 85% yield. Removal of the protecting group completed the synthesis of 7. Another example illustrating intermolecular cyclopropanation can be found in Piers and Moss synthesis of ( )-quadrone 8" (equation 26). Intermolecular cyclopropanation of enamide or vinyl ether functions using ethyl diazoacetate has also been used in the synthesis of eburnamonine 9", pentalenolactone E ester 10" and ( )-dicranenone A11" (equations 27-29). 

Treatment of em-dibromocyclopropanes with one equivalent of alkyllithium at low temperature generates 1-bromocyclopropyllithiums, which have proved to be very versatile intermediates for the synthesis of various cyclopropyl derivatives. Thus, the reactions of the bromolithium compounds with disulfides ketones, alkyl ha-lides , methyleneammonium salts and trimethylsilyl chloride provide the corresponding 1-bromocyclopropyl derivatives (equation 124). A neighbouring ether... 

The classical Simmons-Smith reagent was used to cyclopropanate a double bond in the presence of a triple bond in the synthesis of racemic 11,12-cyclopropyl analogs of hepoxilins Aj and ( )-Undec-2-en-5-ynol (4) was treated with diiodomethane/zinc-copper couple in anhydrous diethyl ether to afford the intermediate racemic (lS, 2S )-l-hydroxymethyl-2-... 

Cyclopropanols can be converted to various cyclopropyloxy derivatives (esters, e.g. acetates, ethers, e.g. methyl and ethyl ethers, and acetals, e.g. tetrahydropyran-2-yloxy derivatives) under the appropriate reaction conditions. In most cases the synthesis of cyclopropyl esters by the reaction between a cyclopropanol and an acid chloride (e.g. formation of 1 ) or acetic anhydride (e.g. formation of 2 ) have been reported. The yields were particularly good (84-95%) when acetic anhydride was used, although a drawback of the reaction can be byproduct formation. When a reactive moiety is attached to the cyclopropane ring in addition to the hydroxy group, other reactions can also occur m-l-(aminomethyl)-2,2-dimethyl-3-(2-methylprop-l-enyl)cyclopropanol (3) reacted with phosgene in benzene to give the corresponding carbamate l,l-dimethyl-2-(2-methylprop-l-enyl)-4-oxa-6-azaspiro[2.4]heptan-5-one (4) in 31% yield. ... 

In a paper published along with that of Stevens and Wentland20 and in agreement with these authors, Keely and Tahk23 reported the independent synthesis of dl-mesembrine, also from I-methyl-3-(3,4-dimethoxyphenyl)-2-pyrroline and methyl vinyl ketone. In their work the cyclopropyl derivative 3b was prepared from the reaction of the anion of 3,4-dimethoxyphenylacetonitrile (lc) with ethylene dibromide in dimethyl sulfoxide and its sodium salt as solvent and base. Reduction with ethereal diisobutylaluminum hydride gave the aldehyde, which was condensed with excess methylamine in benzene-ether solution with calcium oxide as the dehydrating agent. 

The reaction of several a-diazoketones with ethyl vinyl ether under various conditions has been examined. In the presence of metal salts [Rh2(OAc)4, Pd(OAc)2, CuCl] ethoxy dihydrofurans are produced. The initial product of this reaction sequence is a cyclopropyl ketone which suffers from a spontaneous rearrangement to the dihydrofuran <96CJC2401>. An efficient synthesis of 3-acylfurans is achieved by Ag(I)/Celite mediated cycloaddition of dicarbonyl compounds with vinyl sulfide (<97TL5671> see also <97TL2095>). A two-step synthesis of 2-substituted 4-furanmethanol compounds was reported. The method involves a Homer-Wadsworth-Emmons reaction between 3-ketophosphonates and l,3-diacetoxy-2-... 

An effort was made to prepare a butyl analogue containing a ring, but it was never completed. This was the cyclopropylmethyl isomer, 2,5-dimethoxy-4-cyclo-propylmethylamphetamine hydrochloride, DOCPM. Only the first step of its synthesis was complete (the reaction of cyclopropylcarboxylic acid chloride with p-dimethoxybenzene) and even it went badly. The desired ketone (2,5-dimethoxyphenyl cyclopropyl ketone) was most difficult to separate from the recovered starting ether. A promising approach would be the isolation of the phenol (2-hydroxy-5-methoxyphenyl cyclopropyl ketone) which is a beautiful yellow solid with a melting point of 99-100 °C from methanol. Anal. (Cl 1 HI203) C,H. It then could be methylated to the wanted intermediate. It is the major product when the reaction is conducted with anhydrous aluminum chloride in methylene chloride. 

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