Regiospecificity expansion, regiospecific

A similar regiospecific [2 -I- 2] cycloaddition across a C=S group occurred when benzoyl isothiocyanate (436) and 2,3-diphenyl-1-azirine were heated in refiuxing benzene for 12 hours. The product obtained was shown to be (438) and an intermediate such as (437) could also be involved in this cycloaddition (74JOC3763). In contrast, thiobenzoyl isocyanate added in a [4-1-2] fashion, and after ring expansion gave a thiadiazepine derivative. 

Regiospecific ring expansion has been observed on irradiation of the 3-substituted 1-iminopyridinium ylids 92 to give the l//-l,2-diazepines 93.78 Various novel ring systems have been prepared in an analogous fashion. Thus, for example, the previously unknown 1H-1,3-benzodiazepines 94 were obtained by irradiation in dichloromethane of the isoquinoline /V-imides 9579 a possible pathway is outlined in Scheme 2. The novel ring systems, If/-1,3- and 3//-2,3-thieno[2,3-d]diazepines, have similarly been prepared by irradiation of 7-methylthieno[2,3-c]pyridine /V-imides,80 and... 

In work which remains unpublished, Wenkert has succeeded in cleverly transforming 2-methylcyclopentanone into isocomene The key elements of his strategy (Scheme LXXXIII) are the acid-catalyzed ring expansion of methoxycyclopropane 747 to 748 and the regiospecific homologation of the cyclobutanone to 749. Unfortunately, the Wolff-Kishner reduction of this penultimate intermediate affords both 731 and its epimer. 

The key steps in the ring expansion of the cyclobutanones (186) and (787) are the Baeyer-Villiger oxidation effected by H202—HOAc. It is noteworthy to point out that the Baeyer-Villiger oxidation is regiospecific and serves to be an excellent method for the preparation of y-lactone from cyclobutanones. 

Because of our interest in one-pot, multicomponent annulations,7 we envisioned a flexible and efficient protocol which would link the four different components via the formation of four new bonds (a-d, eq. 1) in one reaction vessel. The intermediate 7-hydroxystannanes thus formed in eq. 1 could be oxidatively fragmented8 to produce both ring enlargement and regiospecific formation of an alkenyl unit. This 4-atom ring expansion methodology of common sized a,p-unsaturated ketones has led to the syntheses of many mono- and disubstituted 9-, 10-, and 11-membered unsaturated macrolides (Table ). 

In a base-catalyzed ring expansion route to 1,2-diazocines, Sommelet-Hauser rearrangement of the A-amino derivative of nicotine (22) using sodium in ammonia afforded pyrido-l,2-diazocine 23. The reaction was regiospecific, i.e., no isomeric 24 was detected (82H1595). 

The first 1,3,4-triphosphole 172 was produced by the thermally induced regiospecific insertion of phosphaalkyne P=C-N(Pr )TMS into the P-P bond of the l//-diphosphirene complex 130 (35% yield). Interestingly, a similar ring expansion was not observed with the sterically more bulky diphosphirene complex 87d, which is the precursor of 130... 

The thermal C3 -> 5 ring expansion of 1-siloxy-l-vinylcyclopropanes occurs either from the Z or from the E isomers. It leads to cyclopentanone silyl enol ethers that are able to undergo either further regiospecific alkylation into 2,3-disubstituted cyclopenta-nones 145.209,213 dehydrosilylation into cyclopentenones The overall process constitutes an efficient three-carbon annelation process (equation 129). 

Later, /J-lactams have been obtained by regiospecific metal-catalyzed ring expansion of aziridines. Treatment of A -t ri-butyl-2-phenylaziridine (252) with carbon monoxide in benzene, employing chlorodicarbonylrhodium(I) dimer as the catalyst at 90°C and 20 atm, produced the azetidinone 253 in quantitative yield. Several other )S-lactams were also synthesized from the corresponding aziridine derivatives by the application of this method, and it was noted that this reaction was completely regiospecific. The possible reaction mechanism for the formation of fi-btciams is outlined in Scheme 47. 

The regiospecific carbonylation of oxetanes and thietanes, resulting in a ring expansion... 

Further examples including extrusion and ring expansion reactions <75JCS(Pi)632> as well as intramolecular [3 -I- 2] cycloadditions have been reported <81LA347>. In addition, the mesoionic 1,3-dithiolones (124) react with formaldehyde regiospecifically in a [3 -i- 2] cycloaddition reaction with formation of the 2-oxa-6,7-dithiabicyclo[2.2.1]heptanone derivative (125) (Equation (17)) <78CB3l7l>. Furthermore, upon reaction with the N=N bond of dimethyl azodicarboxylate, (126) is obtained (Equation (18)) <78CB3171>. 

Alkylation of 1 with phenacyl bromide derivatives in the presence of sodium ethoxide gave tricarbonyl 145, which were cyclized to ben-zo[fr]thiophenes 146 upon treatment with La wesson s reagent (99JHC687). Tetrahydrobenzothiophenes 146 with hydroxylamine hydrochloride afforded syn/antioximes 147, which upon treatment with PPA underwent a regiospecific ring expansion to thieno[3,2-c]azepinones 148 (99JHC687). 

Hydrazoic acid (Schmidt reaction) caused expansion of the carbocyclic ring of 4,7,7-trimethyl-2-phenyl-5-oxo-5,6,7,8-tetrahydroquinazoline in a regiospecific manner and gave the fused azepinone 75.200... 

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