Allyl vinyl ether synthesis

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

Among phosphonate esters (170) used in olefin synthesis were those with R = S-CeHi-Br-/ , S02 C6H4-Br-A CO-NHR, and S CHa CEi-XHa. The allyl vinyl thio-ethers (171) obtained using the last of these gave a-allyl-aldehydes on pyrolysis in the presence of red mercuric oxide. 

Cu-catalyzed domino reactions have been used for the synthesis of carbocycles, as well as for heterocycles such as indoles, benzoxazoles, and quinoxalines. A very useful process is also the combination of the formation of allyl vinyl ethers, followed by a Claisen rearrangement. 

The isomerization of allyl ethers and allyl acetals to vinyl ethers or vinyl acetals, respectively, has found many applications in organic synthesis (Equation (17)). Various transition metal catalysts have been reported in the literature for the isomerization of allyl ethers and allyl acetals. 

Sattelkau and Eilbracht90 have exploited the Claisen rearrangement of allyl vinyl ethers in their synthesis of several spiro compounds. As shown below in equation 62, 7,9-dimethyl-l,4-dioxa-spiro[4,5]decan-8-one, 118, was converted to a ,/J-unsaturated ester 119 which was reduced to allyl alcohol 120906. Allyl vinyl ether 121 underwent a rhodium-catalyzed Claisen rearrangement to afford 7r,13r-dimethyl-l,4-dioxa-(8rC9)-dispiro[4.2.4.2]tetradecan-10-one (122) in 36% yield. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

The Claisen rearrangement of allyl vinyl ethers is a classic method for the stereoselective synthesis of y,J-unsaturated esters. The allylic C-H activation is an alternative way of generating the same products [135]. Reactions with silyl-substituted cyclohexenes 197 demonstrate how the diastereoselectivity in the formation of 198 improves (40% to 88% de) for the C-H insertion reactions as the size of the silyl group increases (TMS to TBDPS) (Tab. 14.14). Indeed, in cases where there is good size differentiation between the two substituents at a methylene site, high diastereo- and enantioselectivity is possible in the C-H activation. 

The use of the Claisen rearrangement and several other methods for the stereoselective synthesis of trisubstituted olefins has been reviewed.6 In allyl vinyl ethers of type A, the stereochemistry of the rearrangement is determined largely... 

An aluminium-catalysed tandem Claisen-ene sequence has been developed for the synthesis of homoallylic alcohols (89) and thence a-methylene-y-butyrolactones (90) in good overall yields. Extensive investigation has revealed that Et2AlSPh catalyses Claisen rearrangement of ft -substituted allyl vinyl ethers (85) into 0,5-aluminium... 

The method is an extension of the well-known Grignard synthesis in ethers to the use of nonsolvating media, and is a development of procedures previously reported.2-6 A version of it has been employed with straight-chain primary alkyl chlorides, bromides, and iodides from C2 to Cu,5-7 and in solvents (or an excess of the halide) which permit reaction temperatures above 120°, with simple aryl halides such as chlorobenzene and 1-chloro-naphthalene. Branched-chain primary, secondary, and tertiary alkyl halides, allyl, vinyl, and benzyl halides either fail to react or give extensive side reactions. Better results are reported to be obtained in such cases with the use of catalytic quantities of a mixture of an alkoxide and an ether such as diethyl ether or tetrahydrofuran in a hydrocarbon medium, but the products are not, of course, completely unsolvated.4... 

The Claisen rearrangement was used in the asymmetric total synthesis of (+)-9(ll)-dehydroestrone methyl ether (5), a versatile intermediate in the synthesis of estrogens5 (Scheme 1. If). The key feature of the synthesis is the successful development of the asymmetric tandem Claisen-ene sequence. Thus, a solution of the cyclic enol ether 6 in toluene was heated in a sealed tube at 180 C for 60 hours to afford the product 9 in 76% isolated yield after deprotection of the silyl enol ether. The Claisen rearrangement of the allyl vinyl ether 6 occurred stereoselectively to give an intermediate (7), in which the 8,14-configuration was 90% syn. The stereoselectivity in the Claisen rearrangement can be explained... 

As discussed in Sect. 2, a-selanylalkyllithiums, generated from selenoacetals, can react with various electrophilic reagents, i. e. chloromethyl isopropyl ether for the synthesis of la-hydroxy vitamin D analogues [25] and with propargylic chloride derivatives for the preparation of alkynols [26]. A synthesis of vinyl-cyclopropane derivatives from l,4-dichloro-but-2-ene was achieved with trans stereoselectivity (>93%) in 68-89% yield. This one-pot cyclization, via an intramolecular allylic substitution, required the presence of two equivalents of u-BuLi [26] (Scheme 23). 

A mercury-free route to allyl vinyl ethers that relies on the Michael addition of allyl alcohols to unsubstituted alkenyl sulfoxides, followed by thermal loss of sulfenic acid and concurrent Claisen rearrangement has been described [145]. This methodology has been applied to the synthesis of isocar-bacyclin [146]. Posner reported an acid-catalyzed protocol that produces conjugated dienoate esters from allylic alcohols and a sulfinyl orthoester [147]. Additionally, the use of propargyl alcoholates and a chloro alkenyl sulfox-... 

Mandai, Saito and cowoikers recently described a new synthesis of isocaibacyclin, which features a crucial one-pot, three-step transformation tandem tertiary allyl vinyl ether formation, Claisen rearrangement, and ene cyclization led from alcohol (57) directly to bicyclo[3.3.0]octane (59 heme 10). Clearly, due to improvements in the preparation of Ae allyl vinyl ether moiety, there is a trend even in the classical Qaisen rearrangement to taclde more complex structural challenges successfully. 

Synthesis ofkarahanaenaue. The ionic reaction of NBS in CCI4 at room temperature with a y-ethylcnic tertiary alcohol leads to an -bromotetrahydrofurane. The reaction has been used in a convenient synthesis of karahanaenone (4), a constituent of hop oil, from linalool (1). Thus reaction of (1) with NBS affords 2-mcthyl-2-vinyl-5-(l-bromo-l-methylethyl)tetrahydrofurane (2) in 85% yield. Dehydrohalogena-tion of (2) with collidine at 110 leads to the allyl vinyl ether (3), which immediately... 

Olefin isomerization catalyzed by ruthenium alkylidene complexes can be applied to the deprotection of allyl ethers, allyl amines, and synthesis of cyclic enol ethers by the sequential reaction of RCM and olefin isomerization. Treatment of 70 with allyl ether affords corresponding vinyl ether, which is subsequently converted into alcohol with an aqueous HCl solution (Eq. 12.37) [44]. In contrast, the allylic chain was substituted at the Cl position, and allyl ether 94 was converted to the corresponding homoallylic 95 (Eq. 12.38). The corresponding enamines were formed by the reaction of 70 with allylamines [44, 45]. Selective deprotection of the allylamines in the presence of allyl ethers by 69 has been observed (Eq. 12.39), which is comparable with the Jt-allyl palladium deallylation methodology. This selectivity was attributed to the ability of the lone pair of the nitrogen atom to conjugate with a new double bond of the enamine intermediate. 

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