Allylation of quinones

Allylation of quinones,1 This reaction has been effected with allylsilanes mediated by TiCLt, but allyltin reagents activated by BF3 etherate are now preferred. The allylated hydroquinones can be oxidized to quinones directly by FeCl3. 

Other Carbonyl Allylation Reactions 9.14.3.5.1 Allylation of quinones and dicarbonyl compounds... 

LPDE solution also catalyzes the conjugate allylation of quinones. This is an important reaction in the preparation of biologically active isoprenoid quinones such as vitamin E, vitamin K, coenzyme Qi, and plastoquinones. If 123 is reacted with the allylsi-lane in 5.0 m LPDE for 15 h at 40 °C, allylhydroquinone 124 is obtained in 73 % yield, together with non-allylated hydroquinone 125 (Sch. 63) [113]. 

Other examples of the application of CIDNP spectroscopy to radical additions and substitutions include the self-substitution of quinones in the presence of tertiary aliphatic amines [111], the photoreactions between hexamethyldisilane and quinones [112], and the allylation of quinones via photoinduced electron transfer from allylstannanes [113] (see also Section V.E). Cycloadditions via radical ions are treated in a separate section (V.D.3). 

Simple, direct and regioselective allylation of 1,4-benzoquinones is an important objective in the synthesis of many isoprenoid quinones such as plastoquinone-1 and vitamin Ki, which play important roles in cellular metabolism and photosynthesis. In a new procedure where the use of a strongly coordinating solvent was found to play an important role, it was shown that reaction of 3 equivalents of (3-methyl-2-butenyl)trifluorosilane with one equivalent of 2,3-dimethyl-l,4-benzoquinone in formamide in the presence of 5 equivalents of FeCl3.6H20 for 23 hours at 40°C gave plastoquinone-1 1, in 90% yield. 

AUylation of quinones.1 In the presence of BF3 etherate various allyl tribu-tyltins add to benzo- and naphthoquinones to form a mixture of the corresponding quinones or hydroquinones. An example is the synthesis of ubiquinone (2). 

Pan et al. reported similar reactions of / -quinones to 4-allyl-4-hydroxycyclohexa-2,5-dienone derivatives (Equation (26)).194 Unsymmetrical quinones show high selectivity in the addition of allylindium reagents to the carbonyl group. Indium-mediated allylation of 1,2-diones occurs via a y-coupling fashion to yield ct-hydroxy keto compounds. In some cases of cinnamylation, the corresponding a-coupling products are obtained (Equation (27)).195,196... 

Most examples of quinone dehydrogenations adjacoit to have been earned out on steroidal ketones and are essentially limited to readily enolizable species. Reactions on esters and amides (Table 8) are far less common and, because of their relatively low ease of enolization, require hanh conditions. Thus, unless stabilization of the intermediate carbonium ion is possible, - elevated temperatures and prolonged reaction times are required (Table 8), which increases the incidence of unwanted side reactions. Frequent by-products are those arising as a result of Diels-Alder reactions or Michael addition to the quinone." Allylic alcohols may be rapidly oxidized to aldehydes or ketones under these conditions and requite prior protection. 

Polyprenylation of quinones. Trimethylstannyllithium converts polyprenyl halides (e.g., geranyl chloride, phytyl chloride) into polyprenyltrimethylstannyl compounds in high yield with retention of stereochemistry of the allylic double bond. In the presence of a Lewis acid (BF3 etherate) these tin compounds couple regio- and stereoselectively with 2-methyl-l,4-naphthoquinone. The reaction to form vitamin K (2) is typical (equation I). ... 

Allylsilanes have surfaced as excellent reagents for the allylation of a variety of electrophiles. This reaction was employed as a key step in the preparation of pyranonaphtho-quinone antibiotics. One such example is shown in equation 6369. 

In this case, values of the rates of individual steps may be obtained from measurements of current density potential curves. In addition, measurements of steady-state potentials provide pertinent information. Methods for the determination of the predominating mechanism of the two alternative mechanisms have recently been outlined by Wagner 124) and applied to the hydrogenation of quinone, allyl alcohol, and vinyl acetate by Takehara (125). 

Compounds studied in the latter work are dimethylethylene carbinol, butenediol, acetylacetylene, and DMVC. Coupled with Wagner s theoretical exposition is the experimental paper of Takehara. This author studied the hydrogenation of quinone to hydroquinone on Pt (concluding that the process is electrochemical), and that of allyl alcohol and vinyl acetate on Pt or Pd-Ag, which he concluded proceeded via the non-electrochemical of Wagner s two proposed routes, in both cases using the diagnostic criteria laid down in the preceding paper. 

Isoprenylation of quinones. Evans and Hoffmann have developed a useful route to prenylated quinones, which are natural products involved in various biological processes. The process is illustrated in equation (I) for the synthesis of 2-isopentenylhydroquinone (3). Reaction of the protected quinone (1) with the allylic bromide in the presence of Rieke magnesium (5, 419) affords the epimeric quinols (2). On deprotection the initial product (a) undergoes a facile rearrangement, probably a Cope rearrangement, to (3) in high yield. 

Tabushi et al. [2] reported allylation of 2-methylhydronaphtho-quinone with allyl bromide using CyD catalyst. The yield of target compound with allylation at the C position is increased from 12% to 43% by using 3 CyD at a molar ratioii 5 to 2-methylhydronaphthoquinone. 

Prolonged irradiation of the endo-Diels-Alder adduct formed between duroquinone and trnns,trans-hexa-2,4-diene yields the cage dione (930). Similar reaction of the exo-Diels-Alder adduct of quinone and diene, in which intramolecular allylic hydrogen abstraction by carbonyl oxygen or enone carbon is not impeded, results in the forma-... 

The alkylation of quinones by jr-allyl nickel complexes is another interesting synthetic route to vitamin K and coenzyme Q, the selectivity of this process being remarkably improved relatively to the classical approach based on acid-catalyzed addition of phytol to quinones. 

A series of substituted coumestans 87 were synthesized by Liu, Wang, and coworkers through the formal [3 + 2] cycloaddition of quinone monoketals 85 and the thiomethyl-substituted coumarin 86 (Scheme 39) [138]. The reaction, catalyzed by tin tetrachloride, involves an allylic substitution/intramolecular cyclization/thiol elimination sequence. 

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