Cychzation intramolecular reductive

On the other hand, removal of the silyl group in 22 with fluoride ion and subsequent mesylation afforded 25. Hydrogenation of 25 followed by cychzation in the presence of sodium acetate and subsequent protection of the amino group with Cbz, treatment with ethanethiol in aqueous TFA, followed by dibenzylation, mercuric chloride catalyzed hydrolysis, sodium borohydride reduction and mesylation afforded 26. Selective hydrogenolysis of the carbamate group in 26 followed by intramolecular cyclization afforded the 3,5-di-O-benzyl ether of quinuclidinediol. Removal of the benzyl groups afforded 2. 

This review is devoted to the stereoselectivity of intermolecular (intramolecular cycloadditions are not included) 1,3-dipolar cycloadditions of chiral sugar-derived nitrones. Note that the labile N - O bond in so-prepared cycloadducts can be readily cleaved imder mild reduction conditions, followed by an intramolecular cycHzation of the liberated amino group and the... 

The SnCl2-reduction system has also been apphed in the reduetion of S 2 nucleophilic substitution products 583, alfording more functional quinolines, 4-(substituted vinyl)-quinolines 584, in moderate yields, with several exclusions of the formation of dihydroquinoline derivative 585 (Scheme 4.174). However, using compounds 586 as substrates without a ketone moiety, the ester group can also participate in the intramolecular cychzation, but the subsequent dehydrogenation does not occur and, therefore, tetrahydroquinolin-2-ones 587 were obtained in 51-62% yields (frans form only). From this study, the preference of the activated carbonyl group COR for cychzation has the order R = Me > Ph > O-alkyl. 

An interesting case of 6-endo- versus 5- j o-cylization has been studied by Bombrun and Sageot [129], who investigated intramolecular Mizoroki-Heck cycUzation of enamides in a thiophene series (Scheme 6.50). If the reaction was carried out under Jeffery conditions, then it proceeded nearly exclusively via the 6- iio-cychzation mode however, when run under reductive Mizoroki-Heck conditions (e.g. in the presence of a hydride source), a 5-exo-cyclization is mostly achieved and the lack of a /3-hydrogen in this case leads to product 179. 

Another noteworthy example of a modified Stephens-Castro reaction is the key step in the total synthesis of the natural product oximidine II (5) with distinct anticancer properties (Scheme 9.4) [12]. The Cu acetylide generated in situ from the terminal alkyne 6 using K2CO3 and Cul, underwent intramolecular cychzation followed by partial reduction of the acetylene unit to give the required macrocycUc triene skeleton 7. 

Buchwald described an oxypalladation reaction, followed by a C-H functionalization. This entirely intramolecular reaction is initiated through a 5-exo Wacker-type cychzation of 84. The resulting a-alkyl-paUadium intermediate M provides subsequent C-H activation at the neighboring arene, which allows a paUadium(II) intermediate N bearing a-alkyl and o-aryl substituents, respectively. Reductive elimination provides the C-C bond installation of 85 with the concomitant release of a paUadium(O) catalyst state. Reoxidation imder aerobic conditions, most probably through a palladium(II) -peroxo complex and protonolysis with the acetic add hber-ated in the previous steps, regenerates the original paUadium(II) diacetate catalyst. 

Intramolecular cyclization reactions with ureas and related nitrogen sources usually require a base at least at one stage of the reaction. As a result, aerobic reaction conditions for terminal reoxidation cannot be realized in this type of intramolecular vicinal difunctionalization. In order to demonstrate the general feasibility of such a process, Muniz employed a platinum catalyst in the cycHzation of six-membered annelation products 132 (Scheme 16.33). Aminometallation under platinum catalysis is known to proceed under neutral conditions, and hence without the requirement of a base addition [92]. The resulting intermediate AA is oxidized by copper(II) to a platinum(IV) intermediate AB, which induces reductive elimination to 133. 

As in the previous coverage of this topic, so numerous are the reported syntheses that routes to the racemic alkaloid rintramolecular acylation at C-2 of the pyrrole 1571 made the alkaloid s C-8/C-8a bond. Yuguchi s intramolecular acylation of pyrrole 1572, by contrast, was a more conventional lactam formation involving nitrogen.The cychzation by Helquist and coworkers entailed reductive... 

A successful asymmetric organocatalytic based C=0 reduction with the Hantzsch ester was not reported until very recently. Terada and Toda developed a relay catalysis that combined Rh(II) and a chiral phosphoric acid catalyst in a one-pot reaction (Scheme 32.15). In this reaction sequence, a rhodium carbene (I) forms in the first step and is followed with an intramolecular cychzation to afford carbonyl ylide intermediate II or oxidopyrylium III. These intermediates are protonated by 7 to yield the chiral ion pair between isobenzopyrylium and the conjugate base of 7 (IV). Intermediate IV is further reduced in situ by Hantzsch ester Id to produce the isochroman-4-one derivative 67, which is finally trapped with benzoyl chloride to afford the chiral product 68. Surprisingly, the reaction sequence proceeds well to give racemic product even without the addition of chiral 7, while giving rise to the desired product with high enantioselectivity in the presence of chiral Brmsted acid 7 [38]. 

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