Nucleophiles ring closure with

Nucleophilic ring closure with cyclimmonium salts l,9a-Dihydro-4H-pyrido[2,l-c][l,2,4]triazines... 

Dimethyl-1,2-dithiolylium perchlorate (488) when treated with ammonia at -5 °C underwent nucleophilic ring opening to (489). Subsequent ring closure with loss of H2S gave 4,5-dimethylisothiazole (490) in 85% yield (see Chapter 4.17). This reaction involved an initial attack of the ammonia at the unsubstituted 3-position. A small amount of the 3,4-dimethylisothiazole (491 15%) indicated that some attack of the ammonia also occurred at the 5-position. 

Bromomethyl)biphenyl-2-carbaldehyde (45) undergoes ring closure with arylamines to give 6-substituted 5//-dibenz[c.t,]azepinium bromides 46 via initial nucleophilic attack by the amine at the aldehyde function.92... 

In contrast, aqueous ethanolic ammonia effects initial nucleophilic displacement of bromide, which is followed by ring closure of the benzylamine so-formed at the aldehyde group to give 5//-dibenz[f, f]azepines 48.85 The (1-bromoethyl) aldehyde 47 undergoes ring closure with benzylamine to give 6-benzyl-5-methyl-5//-dibenz[c t>]azepinium bromide (49, R = Me), whereas the (bromomethyl) aldehyde 45 yields a mixture of the quaternary salt 50 and the aldimine 51, which cyclizes in situ to the aziridinophenanthrene 52 in low yield (9%). 

Ring closure with formation of heterocyclic derivatives may occur when a nucleophilic function is present in the starting alkene and it is suitably placed for cyclization [30-43]. Both kind of the mechanistic pathways shown in path a of Scheme 3 and in Scheme 4 may operate, as exemplified by Schemes 5-6 (Y = O, NR). Clearly, only in the first case carbon monoxide is incorporated into the heterocyclic ring (cyclocarbonylation). 

A-Benzylimines have also been reported to react with acrylonitriles under solidrliquid conditions in which the initial anionic intermediate undeigoes an intramolecular nucleophilic ring closure to produce a diastereoisomeric mixture of the pyrrolidine (Scheme 6.23) [30-32], Similar cyclized products have been reported for the reaction of benzylidene-protected a-amino esters and vinyl ketones [33, 34],... 

The formation of cyclopropanes from 7C-deficient alkenes via an initial Michael-type reaction followed by nucleophilic ring closure of the intermediate anion (Scheme 6.26, see also Section 7.3), is catalysed by the addition of quaternary ammonium phase-transfer catalysts [46,47] which affect the stereochemistry of the ring closure (see Chapter 12). For example, equal amounts of (4) and (5) (X1, X2 = CN) are produced in the presence of benzyltriethylammonium chloride, whereas compound (4) predominates in the absence of the catalyst. In contrast, a,p-unsatu-rated ketones or esters and a-chloroacetic esters [e.g. 48] produce the cyclopropanes (6) (Scheme 6.27) stereoselectively under phase-transfer catalysed conditions and in the absence of the catalyst. Phenyl vinyl sulphone reacts with a-chloroacetonitriles to give the non-cyclized Michael adducts (80%) to the almost complete exclusion of the cyclopropanes. 

With [ N2]hydrazinium hydrogen sulfate and potassium hydroxide, the 2, 3, 5 -tri-0-acetyl-l-( N-amino) (3- N) inosine 54 is obtained (Scheme III.29). The reaction follows the same reaction pathway as described in Scheme III.28 addition of the nucleophile at C-6, ring opening between C(6) and N(l), and ring closure with elimination of nitrous oxide and water. This Sn(ANRORC) reaction provides us with an good entry to N-ring-labeled purines. 

Nucleophilic displacement of 2-chloro-3-phenylquinoxaline with methylamine at 100°-150° and with sodium phenoxide in excess of phenol of 100° gives the expected 2-methylamino- and 2-phenoxy-3-phenylquinoxalines.155 2-Chloroquinoxaline and its 3-phenyl derivative undergo ring closure with aminoacetaldehyde dimethylacetal to an imidazo[l,2-a]quinoxaline.136 Nucleophilic substitution of 2-chloro-quinoxaline with hydroxide ion in water is accelerated by cationic micelles and retarded by anionic micelles. These results were correlated with reactions of l-chloro-2,4-dinitrobenzene, and the characteristics of their transition states were discussed.137... 

Polyfluorophenylacetaldehydes show a novel nucleophilic ring closure reaction pentafluorophenylacetaldehyde, treated with sodium hydride in DMF, gives 4,5,6,7-tetrafluorobenzofuran (123, R = H) (13% yield). Under the same conditions, 2-pentafluorophenylaceto-phenone gives the 2-phenyl derivative (123, R = Ph) (76% yield).323... 

Structurally related dienols and acyclic trienols, when reacted in fluorosulfuric acid, give tricyclic ether derivatives in kinetically controlled cyclization.810,811 The stereospecific product formation is rationalized by synchronous internal anti-addition via chair-like conformations of the protonated cyclohexene ring, resulting in ring closure with equatorial C-C bond formation and concomitant internal nucleophilic termination by anti-addition of the OH group [Eq. (5.294)]. Z/E isomerization may be competitive with cyclization. 

The anion formed on nucleophilic attack of the hydrazine stabilizes by fluoride and subsequent HF elimination to give an o ,/3-unsaturated hydrazone, which undergoes an electrocyclic ring closure with HF elimination to yield 5-fluoro-4-trifluoromethylpyrazoles. The single fluorine bonded to C(5) can be exchanged by a wide variety of nucleophiles (88S194 90BAU2338). 

The problem of simple selectivity in the epoxidation of the electrophilic double bond, which is usually stereoselective, but not stereospecific, is directly related to the mechanism of the reaction. In the case of the epoxidation with peroxide anion HOO , the first step is nucleophilic attack at Cp of the conjugated system, followed by ring closure with expulsion of OH. ... 

Nucleophilic addition to the C-C double bond (Michael addition) is followed by ring closure with elimination of the sulfide (R2S). This reaction mode requires an electron-deficient alkene (typically a, 6-unsaturated carbonyl compounds and acrylonitriles) and represents an important synthetic method, see Section 1.1.1.1.2., and Houben-Weyl Vol. Ell, pl44ff. 

A C—N bond is formed during nucleophilic attack by a pyridine-type nitrogen on an cu-haloethylamine side-chain [2663]. When an even weaker base is used in this type of cyclization, a partly reduced imidazole ring is formed [2310]. If other easily displaced halogen atoms are present, ring closure with sodium sulphide or an alkylamine may be accompanied by displacement of the second halogen... 

When a carbon suboxide molecule reacts with two sites of comparable nucleophilicity, ring closure can take place if a favorable mutual spatial orientation of the four reaction centers prevails. For example, carbon suboxide can react with N-aryl-substituted benzamidines to give six-membered ring products quantitatively (38) ... 

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