Diels-Alder reaction vinyl boronates

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

Enyne 112a having a boron moiety on the alkyne gives vinyl boron 113a [Eq. (6.83)], whose carbon-boron bond can be converted to a carbon-carbon bond. Enyne metathesis followed by Diels-Alder reaction of the resulting diene with DMAD gives tricyclic compound 114 [Eq. (6.84)]. ... 

Recently, the first examples of catalytic enantioselective preparations of chiral a-substituted allylic boronates have appeared. Cyclic dihydropyranylboronate 76 (Fig. 6) is prepared in very high enantiomeric purity by an inverse electron-demand hetero-Diels-Alder reaction between 3-boronoacrolein pinacolate (87) and ethyl vinyl ether catalyzed by chiral Cr(lll) complex 88 (Eq. 64). The resulting boronate 76 adds stereoselectively to aldehydes to give 2-hydroxyalkyl dihydropyran products 90 in a one-pot process.The diastereoselectiv-ity of the addition is explained by invoking transition structure 89. Key to this process is the fact that the possible self-allylboration between 76 and 87 does not take place at room temperature. Several applications of this three-component reaction to the synthesis of complex natural products have been described (see section on Applications to the Synthesis of Natural Products ). 

In order to introduce functionality at key positions, an intramolecular Diels-Alder reaction with furan as the diene component was planned. Suzuki coupling of the previously mentioned 105 with furan-3-boronic acid 106 gave compound 107, which was N-tosylated to 108. Construction of the dienophile portion was performed as in the previous case, by addition of vinylmagne-sium bromide to give 109, followed by MnC>2 oxidation to give the cyclization precursor, which was immediately heated at 120 °C in toluene and afforded compound 110, which incorporates an oxygen atom at both the ketone and vinyl chloride positions of welwistatin (Scheme 24). 

This method was used by Hudlicky in a synthesis of the alkaloid narciclasine. The diol 90 was trapped as the usual acetal and reacted as a diene with the nitroso ester Me02C-N0 in a hetero-Diels-Alder reaction to give 91. Suzuki coupling of the vinylic bromide with the right aryl boronic acid gave 92 and the N-0 bond was reduced with Mo(CO)6 to give the advanced intermediate 93 on the way to narciclasine28 94. 

This method was employed in the stereospecific construction of a silyl diene in a synthesis of chlorotri-cholide (Scheme 2.80). The boronic acid partner 2.233 was prepared by hydroboration and hydrolysis. The vinyl iodide was 2.235 prepared by hydroalumination-iodination. The silyl group was included in order to boost stereoselectivity in the later Diels-Alder reaction, but also served to facilitate the synthesis of the vinyl iodide coupling partner. The Suzuki coupling yielded the diene 2.236 with retention of the stereochemistry of both alkenes. Thallium hydroxide was employed as the Lewis base. Thallium-containing Lewis bases have been found to be advantageous in a number of cases, but the toxicity of thallium is a serious concern. 

A few studies reporting an asymmetric Diels-Alder reaction involving vinyl-boronates have been hitherto published. A catalytic asymmetric cycloaddition has been developed using 3- 3-boronyl-propenoyl)-l,3-oxazolidin-2-ones as dienophiles and a chiral titanium catalyst in the presence of molecular sieves [57]. Adducts were obtained as single isomers in good yield and high enantioselectivity (>93% ee) (Scheme 9.25). Cyclopentadiene gave a mixture of endo and exo isomers (-95 5). 

The Diels-Alder reactions of -deficient 2//-pyran-2-ones with vinyl moiety containing dienophiles under microwave and high pressures have been extensively reviewed. The microwave-initiated intramolecular Diels-Alder reaction of an alkynol with a furan (80) formed a cycloadduct (81) that readily converted to 3,4-disubstituted 5-hydroxyindoles (82) in 15-74% yields in a one-pot reaction (Scheme 23). ° The treatment of < rt/io-(trifluoromethanesulfonyloxy)aryl boronic acid pinacol ester (83) with t- or i-butyllithium generates arynes (85) via boronate intermediate (84) at... 

Bicyclo[2,2,2]octanes.—Extensions of the Diels-Alder reaction, still the most important route to this skeleton, concern the use of keten equivalents/ including the novel dienophiles vinyl boronate esters, the better established a-acetoxyacrylonitrile, and the highly regiospecific a-chloroacrylonitrile, the... 

Recently, Yamamoto and coworkers observed a unique form of chemoselectivity in the Diels-Alder reaction of a,P-unsaturated aldehydes and a,P-unsaturated ketones with cyclohexadiene. Upon subjecting a 1 1 mixture of acrolein and ethyl vinyl ketone toward cycloaddition with one equivalent of cyclopentadiene, the bulky tris-pentafluorophenyl boron Lewis acid (C6F5)3B gives primarily the cycloadduct of the a,p-unsaturated aldehyde, and on the contrary, Tf2NH gives mainly the cycloaddition adduct of the a,p-unsaturated ketone (Equation 14) [20]. 

The transmetallation was applied to the preparation of vinyl, alkynyl and aryl-boranes via a tin-to-boron exchange. Noticeable examples include the preparation of organoborane Lewis acids [369], the synthesis of alkynyldihaloboranes and their Diels-Alder reaction with 1,3-dienes [370], the formation of cyclooheptatrienyl(di-propyl)borane [371] and the preparation of 1-benzoborepines [372]. In addition to this, the transmetallation tin-to-boron can be applied to aUylboron reagents as mentioned in Section 6.3.I.2.2. 

Both vinyl- and alkynyl-9-BBN derivatives are effective dieno-philes in Diels-Alder cycloadditions, leading to boron-functionalized cyclohexenes in a selective manner (eqs 37 and 38). Silylated allenylboranes add selectively as allylboranes to aldehydes, a reaction which has been effectively used to prepare the steroid nucleus through a Hudrlik elimination followed by a Bergman rearrangement (eq 39). ... 

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