Ring rearrangement with

Potassium hydroxide/alcohol Aziridine ring rearrangement with N-decarbalkoxylation... 

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

Dibenzoyldiaziridine (130) thermally rearranges with ring opening and benzoyl migration to the dibenzoylhydrazone (138) (67CB142). 

Difluoro-l -vinylcyclopropane undergoes a free radical 1,3 rearrangement with ring expansion yielding difluorocyclopentene derivatives [131 132] (equation 29) Similar, but more complex rearrangement occurs with l,l-difluoro-4-meth ylenespiro[3 2]hexane (equation 30)... 

Carbonylation ot 1-adamantyl triflate in the presence of tnflic acid also gives a derivative of homoadamantane as the result of a similar rearrangement with ring expansion [55] (equation 36)... 

A similar sequence of reactions takes place with the enamlnes of cyclic ketones (55-57) the initially formed unstable cyclobutene rearranges with insertion of two carbon atoms into the ring. A wide variety of cyclic ketones have been allowed to react in this way. For instance, the enamine (75) gave 76 on reaction with dimethyl acetylenedicarboxylate in refluxing toluene (55) and the heterocyclic enamine (77) obtained from dihydro-3-(2H)-... 

Sharpless and Masumune have applied the AE reaction on chiral allylic alcohols to prepare all 8 of the L-hexoses. ° AE reaction on allylic alcohol 52 provides the epoxy alcohol 53 in 92% yield and in >95% ee. Base catalyze Payne rearrangement followed by ring opening with phenyl thiolate provides diol 54. Protection of the diol is followed by oxidation of the sulfide to the sulfoxide via m-CPBA, Pummerer rearrangement to give the gm-acetoxy sulfide intermediate and finally reduction using Dibal to yield the desired aldehyde 56. Homer-Emmons olefination followed by reduction sets up the second substrate for the AE reaction. The AE reaction on optically active 57 is reagent... 

An interesting reaction is the ring enlargement (with rearrangement) caused by primary, but not secondary, amines with appropriately substituted 2-chloromethylquinazoline 3-oxides (Scheme 5). 

The mechanism proposed by Emmons thus corresponds in part to the decomposition of the trialkyl-oxaziranes by ferrous salts. By radical attack on the 7V-alkyl group of the oxazirane, the radical 32 is formed which rearranges with ring opening to 33. Radical 33 propagates the chain by attack on a further molecule of oxazirane. It takes up an H-atom and is decomposed to ketone and ammonia. The aldehyde produced from the M-alkyl group is converted to tar. 

Two meehanisms were presented to explain this base-eatalyzed rearrangement (Seheme 77). Both of these two proposed meehanisms involve nueleophilie attaek by the base whieh leads to ring opening to generate the amine aleohol intermediate. Subsequent ring elosure with eoneomitant loss of the amine followed by tautomerization eompleted the rearrangement. 

The rearrangement with ring contraction probably is the most important synthetic application of the Favorskii reaction it is for example used in the synthesis of steroids. Yields can vary from good to moderate. As solvents diethyl ether or alcohols are often used. With acyclic a-halo ketones bearing voluminous substituents in a -position, yields can be low a tcrt-butyl substituent will prevent the rearrangement. 

The initial series of major tranquilizers consists of alkylated derivatives of 4-aryl-4-hydroxypiperidines. Construction of this ring system is accomplished by a set of rather unusual reactions. Condensation of methylstyrenes with formaldehyde and ammonium chloride afford the corresponding hexahydro-1,3-oxazines (119). Heating these oxazines in the presence of acid leads to rearrangement with loss of water to the tetrahydropyridines. Scheme 1 shows a possible reaction pathway for these transformations. Addition of hydrogen bromide affords the expected 4-bromo compound (121). This last is easily displaced by water to lead to the desired alcohol (122) The side chain (123) is obtained by Friedel-Crafts acylation of p-fluorobenzene with 4-chloro-butyryl chloride. Alkylation of the appropriate arylpiperidinol with 123 affords the desired butyrophenone derivative. Thus,... 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

Blechert s synthesis of the piperidine alkaloid (-)-halosaline (387) by Ru-catalyzed RRM is outlined in Scheme 76 [160]. In the presence of 5 mol% of catalyst A, the ring rearrangement of metathesis precursor 385 proceeded cleanly with formation of both heterocyclic rings in 386. In situ deprotection of the cyclic silyl ether in 386, followed by selective reduction and removal of the to-syl group led to 387. 

Azido-3-methylquinoxaline 1,4-dioxide (293) underwent thermolytic ring contraction with loss of N2 to give a separable mixture of 2-methyl-3//-benzimidazole-2-carbonitrile 1,3-dioxide (294) and its rearrangement product, 3-methyl-3//-2,l,4-benzoxadiazine-3-carbonitrile 4-oxide (295) (PhH, reflux, 10 min 46% and 41%, respectively) the latter product (295) was also obtained from the benzimidazole (294) (PhH, reflux, 30 min 76%) or from the quinoxaline (293) (PhMe, 90°C, 30 min 94%). ... 

A vinyl cation is probably an intermediate in the acetolysis of 6-phenyl-5-hexynyl brosylate, 86. At 80°, despite the inductive effect of the triple bond, the rate of acetolysis of 86 is comparable to that of the saturated analog and yields, besides the acyclic acetate 87, 36% of the rearranged acetate 88 (83). The exclusive formation of the five-membered ring rearranged product with none of... 

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