6- membered reagents

Versatile [3 + 2]-cydoaddition pathways to five-membered carbocydes involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). Palladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n -2-methylene-l,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e. g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopcntane derivatives is accessible. 

The four-membered vinyloxetane 280 is cleaved with Pd(0j and used for allylation a homoallylic alcohol unit can be introduced into the keto ester 281 as a nucleophile with this reagent to form 282[168],... 

Allyl aryl ethers are used for allylation under basic conditionsfh], but they can be cleaved under neutral conditions. Formation of the five-membered ring compound 284 based on the cyclization of 283 has been applied to the syntheses of methyl jasmonate (285)[15], and sarkomycin[169]. The trisannulation reagent 286 for steroid synthesis undergoes Pd-catalyzed cyclization and aldol condensation to afford CD rings 287 of steroids with a functionalized 18-methyl group 170]. The 3-vinylcyclopentanonecarboxylate 289, formed from 288, is useful for the synthesis of 18-hydroxyestrone (290)[I7I]. 

A standard synthetic sequence for building a six membered cyclic ketone onto an existing aromatic ring is shown in outline as follows Specify the reagents necessary for each step... 

Choose which member in each of the following pairs reacts faster or has the more favor able equihbnum constant for reaction with the indicated reagent Explain your reasoning... 

The ACS Committee on Analytical Reagents is comprised of some 15 members from academia, government, and industry (both manufacturers and users of reagents) and meets twice a year at the ACS headquarters in Washington, D.C. Throughout the year other work is carried out by correspondence. Requirements and details of tests are based on pubHshed work, on the experience of committee members in the examination of reagent chemicals, and on studies of test procedures made by committee members. 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared in a variety of substitution reactions. Table 2 shows the rates of substitution compared to thiophene for formylation by phosgene and iV,AT-dimethylfor-mamide, acetylation by acetic anhydride and tin(IV) chloride, and trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235). 

This group of reagents is commercially available in large quantities some of its members - notably lithium aluminium hydride (LiAlH4), calcium hydride (CaH2), sodium borohydride (NaBH4) and potassium boro-hydride (KBH4) - have found widespread use in the purification of chemicals. 

The smallest member, cyclobutadiene, was the objective of attempted synthesis for many years. The first success was aehieved when cyclobutadiene released from a stable iron complex was trapped with various reagents. ... 

When K.yba, Cram and coworkers attempted the synthesis of dibinaphthyl-22-crown-6 from binaphthol and diethylene glycol ditosylate. the reagent ratio was as described either by V or X (Eqs. 3.1 or 3.3). The intent was to make the 22-membered ring. Some of the 11-membered ring was isolated as well (Eq. 3.5). None was apparently observed for the benzo case because the nine-membered ring is less favorable. In short, it is not the intent of the synthesis but the reaction dynamics which determine the product. 

A mechanism of the 1,2-addition of Grignard reagents to ketones has been suggested by Swain involving a six-membered cyclic transition state. Ashby proposed a detailed mechanism as shown in (l)-(2). 

Nitrones are among the most highly studied and useful reagents for the synthesis of five-membered-nng heterocycles. The first fluonnated nitrone, N-methyl-C-(trifluoromethyl)nitrone, was prepared recently and used to introduce Irifluoromethyl groups into such heterocycles... 

The use of azide reagents is also important for the synthesis of cyclic sulfur(VI)-nitrogen systems. The reaction of SOCI2 with sodium azide in acetonitrile at -35°C provides a convenient preparation of the trimeric sulfanuric chloride [NS(0)C1]3 (Eq. 2.16). " Thionyl azide, SO(N3)2 is generated by the heterogeneous reaction of thionyl chloride vapour with silver azide (Eq. 2.17). This thermally unstable gas was characterized in situ by photoelectron spectroscopy. The phenyl derivative of the six-membered ring [NS(0)Ph]3 can be prepared from lithium azide and PhS(0)Cl. ... 

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