Reactions ring expansion

Ring-expansion Reactions. More work has been reported on the formation of seven-membered rings by rearrangement of divinylcyclopropanes. For example good yields of cyclohepta-1,4-dienes were obtained by treatment of P-iodo-aP-unsaturated ketones with the mixed cuprate LiCu(PhS)(2-vinylcyclopropyl), and thermolysis of the adduct mixture so obtained 3-iodocyclohex-2-enone was converted into bicyclo-[5,4,0]undeca-l(7),4-dien-8-one (75%) in this way. Vinylcyclopropyl-lithium 

Pleiadiene has been prepared by ring-expansion of (43), using a two-step procedure. Treatment of (43) with n-butyl-lithium and dichloromethane gave a mixture of bicyclobutane (44) and pleiadiene (total yield 68 %). Thermolysis, irradiation, treatment with silver perchlorate in benzene, and treatment with iodine in CCl (preferred) were all then used to convert this mixture into pleiadiene exclusively (60-80%).  

Ring-expansion Reactions. The thioacetal (41), which had been prepared from cyclohexanone and the lithium derivative of dithiophenoxymethane in 74% yield, rearranged on treatment with cuprous triflate in the presence of a 3° amine to give 2- 

A convenient procedure reported for the preparation of 2,3-benzotropone involves the addition of a dihalogenocarbene to the ethyl enol ether of a-tetralone followed by Ag -catalysed ring expansion and elimination. Bis(dibromocarbene) adducts of 

Tanaka, J. Tsunetsugu, and S. Ebine, Bull. Chem. Soc. Japan, 1975,48, 2395. 

Solvolysis of ll,ll-dibromo[4,4,l]propellane (55 X Br) in acetic acid-sodium acetate gave a mixture of bicyclo[5,4,0]undec-l(7)-en-2-one (36%, also the major product in water-acetone), 6-bromomethylenecyclodecanone (19%), and benzo-cycloheptane (21 %). A mechanism to account for the formation of the first two products involving the strained rra/is-cycloheptene derivatives (56 R — H, Ac, n = 2) was proposed and supported by rearrangement of the labelled dichloropropellane 

The product distribution was found to depend upon the amount of water present in the system, more 6-(bromomethylene)cyclodecanone being obtained with less water present. This observation was explained in terms of capture of the carbonium ion formed from (56 = 2). There would seem to be no examples yet where an intramolecular shift process competes with ring-opening of a cyclopropyl cation. Solvolysis of 10,10-dibromo[4,3,l]propellane in acetic acid-sodium acetate-acetic anhydride gave products formed via the strained bicyclic acetate (56 n = 1, R = Ac). In the absence of acetic anhydride products derived from the corresponding alcohol (56 n = 1, R — H) were obtained as well. Solvolyses of 10,10-dibromo[4,3,l]propell-3-ene in acetic acid-sodium acetate, in the presence and absence of acetic anhydride, and in acetic acid-silver perchlorate, were also reported. Most of the products may have been formed by reaction pathways similar to those observed for (55).  

The other reactions occurring via intramolecular flve-manbered ring intermediates are decarbonylative cleavages with organoruthenium catalysts [119], asymmetric hydrogenations with rhodium catalysts [120], reductive eliminations with rhodium catalysts [121], one-pot preparations of chiral homoaUyUc alcohol or 

7 Applications of Cyclometalation Reactions and Five-Membered Ring Products... 

An interesting way of achieving macrocyclization consists in building-up of bicyclic systems with normal ring sizes at first and subsequent cleavage of the bridge between both rings. This method was applied, for example, by Mahajan and Resck [67] in the synthesis of the 12-membered lactone 115 from 113 via the bicyclic intermediate 114. 

Reactions of this kind are not always carried out under dilution conditions 

The radical ring extension of a-alkyl-p-tin substituted cyclohexanones in the presence of azobis(isobutyronitril) (AIBN) and tributyltin hydride in diluted solution results in good to excellent yields of the ring-extended products [83] (Fig. 5). 

Using this method the 12-membered ketone 117 can be isolated as the only product in 72% yield from 116, 

Enantiomerically pure triene 395, whose chirality stems from that of /3-D-ribofuranosc, was transformed into the chiral pyrrolidine 396 by intramolecular iodoamination. A subsequent RCM reaction gave indolizine derivative 397. Treatment of this compound with nucleophiles afforded mixtures of indolizine and quinolizine derivatives in 

Intramolecular and intermolecular 1,3-dipolar cycloadditions of aziridine-2-car-boxylic esters with alkenes and alkynes have been investigated [131, 132]. Upon heating, aziridine-2-carboxylates undergo C-2-C-3 bond cleavage to form azome- 

Reaction of 3,5-di-f-butyl-l,2-benzoquinone with pyridine cupric methoxychloride and anhydrous ammonia in pyridine gives the imide 159 (-50%) (81JA5795). The lactone dimer 160 is formed in 51% )deld from 3,6-di-f-butyl-l,2-benzoquinone and 3,3-dimethylbut-l-ynyl-lithium (99MI350). 

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Takamizawa et al. developed a general ring-expansion reaction of heterocycles that, applied to thiazolium salts, yields 1,4-thiazines (496, 497) thiamine (220) reacts with dialkyl acylphosphonates (221) to give the tricyclic 1,4-thiazine (222) (498), which is easily hydrolyzed to dihydro-1,4-thiazinone (223) (499) (Scheme 106). In the case of thiazolium slats containing no functional groups (224), 1,4-thiazine derivatives (226) were directly obtained in fairly good yields (Scheme 107). 

Other ring-expansion reactions have already been mentioned in regard to addition reactions leading to pyrrolothiazoles (Section III. 3), which sometimes rearrange to 1,4-thiazines (333, 497). 

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). 

D. Gutsche and D. Redmore, " Carbooyclio Ring Expansion Reactions, Academio Press, New York, 1968, p. 111. 

Treatment of a cyclic ketone with diazomethane is a method for accomplishing a ring-expansion reaction. For example, treatment of cyclohexanone with diazomethane yields cycloheptanone. Propose a mechanism. 

It has been proposed that the ring-expansion reaction is initiated by attack of the base at position 2 of the quinazoline oxide, which is rendered electron deficient by the inductive effect of the (V-oxide group. In fact, 2-(Chloromethyl)quinazolines, compounds that lack this feature, react with amines to give only simple substitution products.1133... 

Keywords Aziridine-2-carboxylic esters. Ring expansion reactions, Azirine carboxylic esters, Aziridine carbinols. Anomalous amino acids... 

Treatment of aziridinecarboxylic esters having an electron-withdrawing substituent at nitrogen with acetonitrile under BF3 catalysis leads to a smooth ring expansion reaction as depicted in Scheme 28 [31]. 

It should be noted that this sequence shown in Scheme 31 is complementary to that depicted in Scheme 30 in which a-hydroxy-j9-amino acids are prepared. This chemistry of aziridine- and oxiranecarboxylic esters and the corresponding ring expansion reactions has been elaborated to a general protocol for the synthesis of j9-hydroxy-a-amino and a-hydroxy-j9-amino acids, respectively [1,411. 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

Since aromatic substitutions, aliphatic substitutions, additions and conjugate additions to carbonyl compounds, cycloadditions, and ring expansion reactions catalyzed by Fe salts have recently been summarized [17], this section will focus on reactions in which iron salts produce a critical activation on unsaturated functional groups provided by the Lewis-acid character of these salts. 

B. By Hydrolysis Reactions.—Details have appeared of the synthesis of dibenzophosphorin oxides (15) from 5-alkyldibenzophospholes, by reaction with methyl propiolate in the presence of water, and of confirmatory syntheses from phosphinic acid chlorides, as shown below. Evidence for the suggested mechanism of the ring-expansion reaction is presented. The hydrolysis of enamine phosphine oxides is an efficient, although somewhat indirect, method for the preparation of j8-ketoalkylphosphine oxides (16) [see Section 3(iii), for the preparation of enamine oxides]. Reasonable yields (48—66%) of trialkylphosphine oxides (17) have been obtained by the alkaline hydrolysis of the products from the pyrolysis at 220 °C of red phosphorus with alkyl halides, in the presence of iodine. 

BanweU, M.G., Berak, M., Hockless, D.C.R. (1996) Construction of the Colchicine Framework via Two Consecutive Cyclopropane-Mediated Ring-Expansion Reactions. Journal of the Chemical... 

The synthetic utility of the ring expansion reaction was demonstrated by its application to the synthesis of thermolabile thiepins. When the diazo compound (66) obtained from benzo[c]thiopyrylium salt 65 was treated with palladium catalyst under the same conditions as in the case of 63, the product isolated was ethyl 2-naphthoate (68)48). The plausible reaction pathway is one comprising i) decomposition of 66 to the corresponding carbene intermediate, ii) ring expansion to the... 

Scheme 2.64. CsF-initiated ring-expansion reactions of substituted pyridines 2-272 and 2-275.

Scheme 7.1. Domino asymmetric epoxidation/ring expansion reaction.

Scheme 7.2. Asymmetric expoxidation/ring-expansion reaction using chiral (salen)Mn111 complex 7-6.

Functionalized silacyclobutanes 16 result from photochemical decomposition of [azido-, isocya-nato- and isothiocyanato-bis(tert-butyl)silyl]diazoacetates 15. They undergo a remarkably facile ring-expansion reaction to cyclic O-silyl ketene acetals 17 even at 60°C. 

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