Ring-closure of

Ring-closure of Ce. The reaction of (13) with LDA led to a 2,4-diamino-thiophen, which according to n.m.r. exists in the imino-form (14). Compounds of type (14a), prepared by the condensation of methyl aryl ketones with phenyl isothiocyanates in the presence of sodium hydroxide followed by alkylation, gave the aminothiophens (14b) if is COPH or COMe, without additional catalysis by a base. On the other hand, if R is C02Et or CONH2, the thiazolidones (14c) were obtained and if R is CH2Br, (14d) was the product. 

Ring-closure of C2SC2. AUyl 1,2,2-trichlorovinyl sulphides (15) gave a mixture of (16) and (17) upon heating to 100 - nO C.  

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Thus, to name just a few examples, a nucleophilic aliphatic substitution such as the reaction of the bromide 3.5 with sodium iodide (Figure 3-21a) can lead to a range of stereochemical products, from a l l mbrture of 3.6 and 3.7 (racemization) to only 3.7 (inversion) depending on the groups a, b, and c that are bonded to the central carbon atom. The ring closure of the 1,3-butadiene, 3.8, to cyclobutene... 

Note 5. At room temperature the 3,3-sigmatropic rearrangement begins. The anmonia, still present during the work up, will cause ring closure of the allenic dithioester to a 2-ff-thiopyran derivative. 

A very mild and efficient synthesis of N-substituted -lactams uses the Mitsunobu reaction (see section 2.6.2) for the ring closure of seryl dipeptides protected at the terminal N as 4,5-diphenyloxazol-2(3f/)-one ( Ox ) derivatives (see section 2,6.3)... 

Ring opening and further ring closure of 2-imino-oxythiolan-l,3 derivatives (32) by water and/or methanol lead to the corresponding A-4-thiazoline-2-one (26) (Scheme 14) (30-32). 

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

CONHj, COjEt, were also prepared by ring closure of thioamides such as RiCONHCH(R2)C(=S)NH2 with polyphosphoric acid (2 hr at 120°C) (718). 

Base promoted ring closure of vicinal halohydrms... 

The following section describes the preparation of epoxides by the base promoted ring closure of vicinal halohydrms Because vicinal halohydrms are customarily prepared from alkenes (Section 6 17) both methods—epoxidation using peroxy acids and ring closure of halohydrms—are based on alkenes as the starting materials for preparing epoxides... 

Alkylphenols can be synthesized by several approaches, including alkylation of a phenol, hydroxylation of an alkylbenzene, dehydrogenation of an alkylcyclohexanol, or ring closure of an appropriately substituted acycHc compound. The choice of approach depends on the target alkylphenol, availabihty of the starting materials, and cost of processing. The procedures discussed herein encompass commercial methods, general methods, and a few specific examples of commercial interest. 

Ring closure of o-benzoylbenzoic acid to anthraquinone is an unusual reaction in that normally it is not predicted to occur ortho to a keto group. Several theories have been proposed to explain the mechanism whereby this could possibly occur. One involves a complex ionization of o-benzoylbenzoic acid (41), the other favors the intermediate formation of 3-hydroxy-3-phenyl-l(3JT)isobenzofuranone (9) [64693-03-4] and 3-phenylphthaHdyl sulfate (10) (42) ... 

A second synthesis of cobyric acid (14) involves photochemical ring closure of an A—D secocorrinoid. Thus, the Diels-Alder reaction between butadiene and /n j -3-methyl-4-oxopentenoic acid was used as starting point for all four ring A—D synthons (15—18). These were combined in the order B + C — BC + D — BCD + A — ABCD. The resultant cadmium complex (19) was photocyclized in buffered acetic acid to give the metal-free corrinoid (20). A number of steps were involved in converting this material to cobyric acid (14). 

Epoxide formation from chlorohydrins is marked by an increase in rate with alkyl substitution (28) as shown in Figure 1. This phenomenon has been explained on the basis that steric crowding ia the chlorohydrin is somewhat reheved as the epoxide is formed, so that the greatest rehef of strain results from ring closure of the most crowded chlorohydrin (28). 

The photochemical ring closure of certain stilbenes, eg, the highly methyl substituted compound (2) [108028-39-3], C22H2g, and their heterocycHc analogues is the basis for another class of photochromic compounds (31—33). 

Ainino-3-chloroanthraquiQone [84-46-8] (68) is prepared from 2,3-dichloroanthraquiQone by partial chlorine replacement by a NH2 group. 2,3-Dichloroanthraquiaone [84-45-7] (67) is prepared by Friedel-Crafts reaction of phthaUc anhydride and 1,2-dichlorobenzene followed by ring closure of the resultant benzoylbenzoic acidia sulfuric acid (94). 

The ring closure of 1,4-enediones has also been used for furan synthesis even though the precursor is at the wrong oxidation level. The examples in Scheme 14 indicate some of the ways in which the necessary reduction has been effected. 

An alternative method involves reaction of an a-acylaminoketone (12) with a primary amine and subsequent ring closure of the resultant Schiff s base (13) with phosphoryl chloride. This enables the introduction of a 1-substituent as in (14) to be carried out efficiently, and if the amine were replaced with a monosubstituted hydrazine, the imidazole derivative (IS) resulted (78LA1916). 

The role of the 1,1-bielectrophile in ring closures of this type is to provide a one-carbon unit (or heteroatbm) to close the cycle. Thus, the synthesis of the four-atom precursor with two nucleophilic centers 1,4 to each other is an appreciable challenge, especially to obtain a heterocycle at the desired oxidation level. The examples below illustrate the way this approach to synthesis may be gainfully utilized. 

Numerous examples of the ring closure of a binucleophilic system with a 1,1 -bielectrophile leading to five-membered heterocycles with three or more heteroatoms have been described, the popularity of this route no doubt reflecting the comparative ease with which the penultimate product may be obtained. 

Substitution of the nitrogen atom in (289) and subsequent ring closure of (293) under acid cyclodehydration conditions gave the mesoionic system anhydro-5-hydroxythiazoIium hydroxide (294). These reactions are analogous to the cyclodehydration of the A-nitrosogly-cines (295) with acetic anhydride to give the sydnones (296) (see Chapter 4.21). 

The important synthesis of pyrazoles and pyrazolines from aldazines and ketazines belongs to this subsection. Formic acid has often been used to carry out the cyclization (66AHQ6)347) and N-formyl-A -pyrazolines are obtained. The proposed mechanism (70BSF4119) involves the electrocyclic ring closure of the intermediate (587) to the pyrazoline (588 R = H) which subsequently partially isomerizes to the more stable trans isomer (589 R = H) (Section 4.04.2.2.2(vi)). Both isomers are formylated in the final step (R = CHO). 

Although formally it could be classified with the ring transformations (Section 4.04.3.2.2), conversion of 2,4-diphenyl-l,3,4-oxadiazol-2-one (593) by flash vacuum pyrolysis at 500 °C into 3-phenylindazole (595) involves a C(3)—C(3a) ring closure of the diphenylnitrilimine (594) (79AG(E)721). 

Anthra[l,9]pyrazol-6(2//)-one (735) dyes have also been described in the literature (B-70MI40403). TTiesc compounds result by acid ring closure of 1-anthraquinonyl hydrazine or its V-sulfonic acid. 

J E G E R Tetrahydrefuran synthesis Free radical ring closure of alcohols with F%> AcO)41<> tetrahydrofurans... 

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