Cyclisation acid-catalysed

H-Pyran-2-iminium salts (2) are formed by the acid-catalysed cyclisation of the pentadienal (1), itself accessible from the addition of HCl to 5-dimethylaminopenta-2-en-4-ynal, a push-pull enyne <96HCA192>. 

Problem The synthesis of bicycllc lactone (25) by the acid-catalysed cyclisation of (24) could be tried if (23) can be made by via Diels-Alder reaction. 

Furans occur widely in nature and many are important commercially. Thus alcohol (29) is used in various insecticides. The carbon atoms. joined to the ring oxygen atom are at the carbonyl oxidation level so that (29) can be made by acid-catalysed cyclisation of (30). Analysis... 

The two saturated rings of the cannabinoid nucleus are formed with the correct stereochemistry at C-6, C-6a and C-lOa but without selectivity at C-9 in a single acid-catalysed cyclisation step from the phenol 51. Stabilisation of a cationic intermediate by the alkyne group is proposed to account for the facile cyclisation . 

A facile route to l,3,4-oxadiazepin-2-ones has been developed by Komatsu et al. involving acid-catalysed cyclisation of carbazate derivatives derived from the reaction of N, A -di-tert-butyldiaziridinone and a-hydroxy ketones with BF3.Et20 catalysis <00H(52)541>. 

The furo[3,4-ri]oxazoie 54, constracted by cyclisation of a rhodium carbenoid, is a useful compound for the production of unusual benzoxazoles for example, cycloaddition of Al-phenylmaleimide gave the benzoxazole 55 <98JOC7680>. Simpler benzoxazole syntheses include the base catalysed cyclizsation (with loss of the trifluoromethyl anion) of the imines 56 <99TL4119> and the acid catalysed cyclisation of diacylated aminophenols 57 <99H(51)979>. ... 

Formation of the isochroman system is considered to trigger the synthesis of the dibenzopyran (17, X = H2) by the acid catalysed cyclisation of c/s-enediynes (16, X = H2). In a similar manner, the carboxyl function in (16, X = O) promotes cylisation to a dihydropyranone derivative which is followed by a Myers cycloaromatisation to the dibenzopyranone (17, X = O) (95TL9165). 

Fig (6)The transformation of the tetralone (42) to the ester (45) is described. Its tranformation to the tricyclic ketone (46) involves aromatization, subjection to Grignard reaction and intramolecular cyclisation and oxidation. Its conversion to diazomethylketone (48) is carried out in three steps and this on acid catalysed cyclisation yields the enedione (49), which is converted to methy ( )-methylpisiferate (5) by standard organic reactions. 

Substituted pyran-4-ones can be prepared from isoxazoles through reductive cleavage with Mo(CO)6 and acid-catalysed cyclisation of the generated enaminoketone <02TL3565>. 

Indole-fused, or indole-benzo-fused azepinone derivatives have attracted synthetic attention and examples include the preparation of 85 in 84% yield from 84 by intramolecular Heck coupling [01SL848], as well as the preparation of paullone 87 (a CDK inhibitor) by cyclisation of 86 under basic conditions borylation/Suzuki coupling technology was used to access 86 [02JOC1199]. Acid-catalysed cyclisation with polyphosphoric acid was used to prepare the racemic reduced azepino[4,5-6]indoles 92a,b from the precursors 91, which were obtained in turn from CDl-mediated coupling of 88 and 89, followed by reduction of the amide with lithium aluminium hydride [01H1455]. 

Both enantiopure 1-substituted isochromenes and isochromans can be obtained from homophthalaldehyde through initial conversion to a perhydrobenzoxazine by reaction with (-)-8-(benzylamino)menthol. Acid-catalysed cyclisation of the separated diastereomeric alcohols arising from reaction with various organometallic compounds leads to isochromenes in toluene but to the isochroman in lower boiling alcohols (Scheme 15) <06EJO5110>. 

Dibenzothiepines 71 can also be accessed readily from thianthrene 70 by lithiation and subsequent reaction with electrophiles, hydrolysis and acid-catalysed cyclisation <03T2083>. 

The occurrence of 3 can be explained by the acid-catalysed cyclisation of the Claisen rearrangement product 3 [2,3]. The formation of 4 is presumably caused by acid-catalysed isomerisation of 2. 

A similar polymer 119 was prepared by Goldfinger and Swager by an acid-catalysed cyclisation of a PPP precursor 120 with alkyne sidechains (Scheme 54) [163]. There is no report on the emission from this material, but... 

A range of derivatives of 2-nitroacridine has been obtained by the acid catalysed cyclisation of 2-arylamino-5-nitrobenzaldehydes which probably proceeds through protonation of the carbonyl group (J. Rosevear and J.F.K. Wilshire, Austral. J. Chem., 1981, 839). 

The acid catalysed cyclisation of 2-acety1-3-arylamino-1,4-naphthoquinones (111) gives benz[b]acridine-6,11-... 

Quatemization of the i ridylnaphthalene derivatives (132) followed by reduction affords a mixture of diastereoisomers of which only one undergoes acid catalysed cyclisation to the partially reduced cis-fused naphtho[1.8-f.g]quinoline (E. Reimann and G. Bauer, Arch.Fharm.. 1984, 317, 517). The C-nmr shifts for the angular methyl group lie between 6 28 and 35 in the six... 

In the ester series the acid-catalysed cyclisation of the alcohol (452, R=C00Me) gives a mixture of the By-unsaturated ester (455, R=C00Me) and its aB-unsaturated isomer, both of which give the same tropolone on oxidation and this yields (456) on hydrolysis and decarboxylation. 

The failure of the usual acid-catalysed cyclisation methods to furnish specific methoxy substituted heteroaromatic compounds may be illustrated with the synthesis of 8-methoxy isoquinoline by Bischler-Napieralski reaction. In this, the starting compound is the acyl derivative of meta methoxy p-phenylethylamine. This, in the acid catalysed ring closure reaction, furnishes the 6-methoxy isoquinoline (5) rather than the 8-methoxy isomer (6). 

To obtain the linear pyranoquinoline the 2-OCH3 of 94 has to be demethylated to obtain 96. Interestingly 96 also gives only the angular compound 97 either by thermal or acid catalysed cyclisation. However acid catalysed cyclisation, in the cold, in presence of UV radiation (to convert the trans to cis olefin) gives the linear compound 98... 

If the diamine 204 is wanted, hydrolysis of the amide in 217 is the answer. But if the more interesting imidazolo-pyridine 205 is wanted, acid catalysed cyclisation of the amine onto the amide gives the imidazole 205 R = Me directly. Of course if R needs to be something else, the original acylation of 213 should be done with that anhydride (RCO)20. 

Selectivity in the epoxidation of dienes where the two alkene residues are a widely differing nucleophilicities is not difficult to achieve and we will return to this point later in this review. However, it is worth pointing out that quite good diastereofacial selectivity was reported in the epoxidation shown in Eq. (12), part of a total synthesis of (+ )-altholactone. The epoxidation using MCPB A proceeded with poor selectivity whereas MMPP gave predominant attack on the P-face and a 3.5 1 mixture of the substituted tetrahydrofuran derivatives was obtained after acid catalysed cyclisation [26]. 

In this extraordinary reaction, quinoline is produced when aniline, concentrated sulfuric acid, glycerol and a mild oxidising agent are heated together. " The reaction has been shown to proceed via dehydration of the glycerol to acrolein, to which aniline then adds in a conjugate fashion. Acid-catalysed cyclisation... 

The ring synthesis of five-membered heterocycles has been extensively investigated, and many and subtle methods have been devised. Each of these three heterocyclic systems can be prepared from 1,4-dicarbonyl-compounds, for furans by acid-catalysed cyclising dehydration, and for pyrroles and thiophenes by interaction with ammonia or a primary amine, or a source of sulfur, respectively. 

The adduct (276) from j9-pinene (241) and acrolein undergoes an acid-catalysed cyclisation giving 6-isopropyltetralin and the tricyclic diol (277), the latter being favoured at low acid concentrations. The enamines of pinocamphone (278) and verbanone (280) can be alkylated, and when methyl vinyl ketone is employed, the product is a tricyclic ketone [e.g. (279), from the pinocamphone]. ... 

The complete chemical and. ST-ray studies of laurinterol (140) have been published. In addition to laurinterol, debromolaurinterol and isolaurinterol (141) are also found in Laurencia intermedia Yamada. Aplysin (142), isolated from Aplysia kurodai Baba, can be derived from laurinterol by acid-catalysed cyclisation. A synthesis of both aplysin and debromoaplysin, involving suitable elaboration of the alcohol (143) has been reported.The synthesis of the methyl... 

Jain and McCloskey have found that acid-catalysed cyclisation of costu-nolide (234) on Amberlite cation exchange resin gives a good yield of a- (235) and ]8-cyclocostunolides (236). Furthermore, the same authors have shown not only that dihydrocostunolide (237) undergoes the normal Cope rearrangement but also that at elevated temperatures the bicyclic compounds (238), (239), and (240) are obtained. 

Spectral studies have shown the configuration of the major (377) and minor (378) A-nor-B-homosteroids formed in the acid-catalysed cyclisation of (376), to be as shown. ... 

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