Ether formation Fragmentation

Expulsion of nitrogen with formation of the A -l-methyl compound (9) occurs by heating (8) at ca. 220° or at room temperature by contact with acidic adsorbents. ° However, in this case perchloric acid or boron trifluoride etherate catalyzed fragmentation is not possible, although high yields (80 %) of (9) are obtained by heating (8) with quinoline or aniline. The la,2a-methylene compound (10) is always obtained as a by-product in 5% yield. 

Details of the Janda-Chen synthesis were as follows. A tetrahydropyran (THP) linker was attached to the NCPS support enabling attachment of alcohols via THP ether formation.13 The THP-NCPS resin 1 is derivatized with / -(+)-4-hydroxy-2-cyclopentanone 2, giving the THP ether-based resin 3, followed by coupling of the C13 20 fragment by enone-cuprate addition. The cuprate required was generated from the corresponding E-vinyl stannane 4. The resulting enolate was trapped as the silyl end ether... 

How do reduction reactions that lead to a-CH2-groups affect pulping efficiencies Several typical (P-aryl ether) cleavage pathways would be blocked, which would have a negative effect on efficient delignification. However, because quinone methide formation would be prohibited, undesirable vinyl ether formation and condensation reactions would also be blocked. Bulk-phase fragmentation reactions, involving a... 

One of the difficulties in establishing the detailed chemistry of initial phase pulping is the dominance of the delignification rate by the first step in the process— formation of a quinone methide intermediate. The rates of subsequent steps are difficult to determine however, the rates of these steps are critical to the partitioning of the QMs toward fragmentation processes (p-aryl ether cleavage) as opposed to undesirable competing reactions, such as vinyl ether formation and condensation reactions [57]. 

There are few examples of formation of simple azetidines from [2 + 2] fragments, although this type of approach is important for certain azetidine derivatives, e.g. azetidin-2-ones (see Section 5.09.3). Nitrogen analogues of the Paterno-Buchi route to oxetanes are rare an example involves the photoaddition of 3-ethoxyisoindolone (37) to the enol ether (38) (75JA7288, 72CC1144). 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

From intermediate 12, the path to key intermediate 7 is straightforward. Reductive removal of the benzyloxymethyl protecting group in 12 with lithium metal in liquid ammonia provides diol 27 in an overall yield of 70% from 14. Simultaneous protection of the vicinal hydroxyl groups in 27 in the form of a cyclopentanone ketal is accompanied by cleavage of the tert-butyldimethylsilyl ether. Treatment of the resultant primary alcohol with /V-bromosuccini-mide (NBS) arid triphenylphopshine accomplishes the formation of bromide 7, the central fragment of monensin, in 71 % yield from 27. 

A retroaldol fragmentation subsequent to the addition of p-TsOI I and a small amount of water to epoxide 206, obtained by oxidation of enol ether 205 with DMDO, resulted in the direct formation of dialdehyde hydrate 208, possessing the spirostructure necessary for the construction of the fused-rings core of ( )-ginkoli-de B. Apparently, hydrolysis of the epoxide produces the hemiacetal 207, which undergoes retroaldol fragmentation of the cydobutane to afford the dialdehyde, which forms the stable hydrate 208 (Scheme 8.52) [94]. 

Finally, the in situ formation and thermolysis of trialkyltin ethers derived from tertiary homoal-lylic alcohols has been developed into a useful allylstannane synthesis39,40. This fragmentation is the reverse of the addition of an allylstannane to a ketone41. 

The other fragmentation pathways are typical for diaryl sulfoxides1-4-6,1. A corresponding ortho effect was found in chlorodiphenyl ethers and sulfides but not in sulfones12 (12) were the sulfinate ester rearrangements1-4,6,11 and the consequent formation of the m/z 125 and m/z 159 ions suppress the other possible fragmentations of the molecular ions (equation 4). It is also noteworthy that the ratio [m/z 125] [m/z 159] increases with increasing distance between the chlorine and the sulfur (equation 4). 

The suggested fragments from (54a) are a carbonyl compound (58) and a peroxy zwitterion (59), the latter then effecting a 1,3-dipolar addition on the former to yield the ozonide (57a). Alternative reactions of the zwitterion (59), including its polymerisation, lead to the formation of the abnormal products that are sometimes observed in addition to the ozonide, If ozonolysis is carried out in MeOH as solvent then (59) is trapped , as it is formed, by its conversion into the relatively stable a-hydroperoxy ether (60) ... 

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