Reactions ring-closing

Entropy of activation is also responsible for the difficulty in closing rings larger then six membered. Consider a ring-closing reaction in which the two groups that must interact are situated on the ends of a 10-carbon chain. In order for reaction to... 

A ring closing reaction involving coordinated amide groups is observed for (159a), which forms (159b) upon condensation with formaldehyde and methylamine.549... 

Ring-closing reactions promoted by mercuric salts are valuable transformations which find an increasing use in the total synthesis of various natural products.130-140 Several examples of solvomercurations demonstrating the applicability of these transformations to the synthesis of natural product precursors are presented in Table 2. Piperidines (entry a), 141 pyrans (entries b-d), 142-144 and furans (entries e, f)14S>146 have been obtained in good yields and diastereoselectivity. These derivatives serve as starting materials for various natural products and can be demercu-rated under reducing conditions.147... 

Table 2.1 Summary of in situ ring-closing reactions REAGENTS PRODUCTS...

In the study of substituent effect on the thermal electrocyclic ring-closing reactions of vinylallenes, two stereo-isomeric boryl-substituted vinylallenes 13 and 14 are synthesized by means of the palladium-catalyzed stannaboration of alkynes (Schemes 59 and 60).248... 

The six or seven-membered cyclic ethers that comprise the Brevetoxin subunits could be prepared by Mo-catalyzed RCM (Eq. 42) [13]. It is noteworthy that the most successful substrates were those in which R, =H and R2=Me. The first metal-containing intermediate in that circumstance therefore is likely to be that in which there is an oxygen bound to the alkylidene carbon atom yet that alkyli-dene behaves in a ring-closing reaction in the expected manner. 

Deprotonation of Alkoxyallenes and Reaction with Electrophiles -Ring-Closing Reactions... 

Subsequent Ring-Closing Reactions of the Primary Allene Adducts... 

Clipping. The clipping method relies on the synthesis of the rotaxane by formation of the macrocycle (host) in the presence of a dumbbell shaped molecule. The partial macrocycle undergoes a ring closing reaction around the dumbbell shaped molecule, thereby forming the rotaxane. 

Table 3 lists PAEs containing Si, Fe, and B. Silole containing Si in the five-mem-bered ring [157] has a low transition energy because of its low LUMO level [158], and its homopolymers have been prepared [159, 160]. 2,5-Dili-thiosiloles, which can be prepared by the ring-closing reaction of diethynyl-silane, serve as starting materials for various 2,5-disubstituted siloles such as 2,5-dibromosiloles and 2,5-di(trialkylstannyl)siloles [161,162]. PAEs with the silole ring have been reported as shown in Nos. 1-4 in Table 3. PAE-type polymers with Si atoms in the main chain have also been prepared (Nos. 12-16), and their optical properties including photoconductivity have been revealed [155,156].

This chelation-controlled addition generated the homoallylic alcohols 86 with a diastereoselectivity of syn anti=S6ll4 , In contrast to the other syntheses described so far, the ring closing reaction was a 5 2 reaction but not a... 

In 1970 ring-opening and ring-closing reactions of 2,2-dialkyl-1,3,2-dioxa- and -1,3,2-oxathia-stannolanes with carbon disulfide and phenyl isothiocyanate involving nucleophilic attack at the tin atom were discovered <70CC235>. Since these reactions are described as proceeding via a cyclic transition state they will be considered in Section 4.24.2,5.10. 

Different ring sizes exhibit distinct characteristics, especially in the case of smaller cyclic peptides (di- to hexapeptides), including ring-closing reactions, which will be treated briefly, and related conformational characteristics. 

Scheme 15.5 Illustration of the formation of the trans- and a.v-ring-closed isomers of 34 by electrochemical ring-closing reaction.

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