Propenyl aldehyde

The main isomerization products of 2,3-dihydrofuran are cyclopropane carboxalde-hyde (reaction (1)) and propenyl aldehyde (reaction (2)) [76,77]. Propenyl aldehyde can be obtained also via isomerization of the product—cyclopropane carboxaldehyde (reaction (3)). Reactions (1) and (2) go directly from the reactant to the product via one transition state. Reaction (3) has one intermediate and two transition states. The transition state for the production of cyclopropane carboxaldehyde (reaction (1)) involves a C(sp )—O bond cleavage and formation of a new C=0 double bond with the second adjacent carbon atom, together with a C—C bond formation (Figure 6.12). 

The isomerization of cyclopropane carboxaldehyde to propenyl aldehyde involves an opening of the cyclopropane ring and H-atom shift to form a methyl group. The pathway for this process is very similar to the one that has been found for cyclopropane carbonitrile that has been described in Section 6.2.2. 

Further functionalization has been carried out in the mixed benzo-binaphtho crown shown in Eq. (3.55). Using 2-allylcatechol as starting material, the mixed crown was prepared in the usual fashion. The allyl group was isomerized to a propenyl substituent by treatment with potassium f-butoxide in a benzene/f-butanol mixture. Selective ozono-lysis affords the aldehyde. 

Furthermore, being a member of the family of nonprostereogenic allylmetals, 2-(alkoxy- or alkylaminocarbonyl)-2-propenyl reagents offer the possibility of introducing the a-methylene-propanoic acid /f-enolate to aldehydes. 

In ( )-[2-(l-propenyl)-l, 3-dithian-2-yl]lithium, no problem of EjZ selectivity arises. It is easily prepared by deprotonation of the allylic dithiane87,88 with butyllithium in THF, whereas deprotonation of the 2-propylidene-l, 3-dithiane requires the assistance of HMPA. The addition to saturated aldehydes proceeds with excellent y-regioseleetivity and anti selectivity88,89. As often observed in similar cases, aldehydes which bear an, p2-carbon atom adjacent to the carbonyl group give lower selectivities. The stereoselectivity decreases with ketones (2-bu-tanone y/a 84 16, antiisyn 77 23)88. The reaction with ethyl 2-oxopropanoate is merely nonstereoselective90, but addition of zinc chloride improved the syn/anti ratio to 96 4, leading to an efficient synthesis of ( )-crobarbatic acid. 

High -/-selectivity in the addition of aldehydes and ketones is also reported for [1,1,2-tris-(phenyllhio)-2-propenyl]lithium91 and [l,l-bis(isopropylthio)-2-methyl-2-propenyl]lithium92. No selectivity is observed with ( )-2-lithio-2-(2-phcnvlcthcnyl)-l,3-dithianc93, but essentially complete a-selectivity occurs in the presence of boron trifluoride-diethyl ether. 

The nucleophilic alkenoylation of a-phenyl-substituted aldehydes and ketones with (methyl-substituted) [l-cyano-l-(trimethylsilyloxy)-2-propenyl]lithium proceeds with good 1,2-induction to afford the j> i-hydroxy ketones109. 

To a solution of 5 mmol of 1,3-diphcnyl 3-[(S )-2-mcthoxymethyl-l-pyrrolidinyl]-2-propenyl[lithium in 10 mL of tort-butyl methyl ether (prepared according to Section D. 1.1.1.2.2.3.) at 0°C. 6.25 mmol of the aldehyde (and eventually 6.25 mmol of lithium halogenide in 27 mL of leri-butyl methyl ether) are added dropwise. Stirring is continued for 2 h and 0.39 g (5.0 mmol) of acetyl chloride are added. After 2 h stirring at r.t., 10 mL of the solvent, 50 mL of diethyl ether and 10 mL of 2 N aq hydrochloric acid are added and stirring is continued for 2 h at 20 C. The organic layer is extracted with three 20 mL-portions of water and the aqueous solutions are reextracted with diethyl ether. The combined aqueous solutions are dried over Na,S04, concentrated in vacuum and the residue distilled to yield a mixture of xyn- and on/i-ketones >90% ee, determined by H-NMR with Pr(hfc)3. 

A mixture of 1.97 g (10 mmol) of (Z)-2-(3-methoxy-2-propenyl)-4,4,5,5-tetramethyl-l, 3,2-dioxaborolane and 755 mg (13 mntol) of propanal is stirred without solvent for 2d at r.t., and then for 3 h at 50LC. Residual aldehyde is removed in vacuo then the residue is dissolved in 3 inL of CH,C1, and treated with 1.5 g (10 mmol) of triethanolamine for 12 h. The mixture is filtered, concentrated and bulb-to-bulb distilled from a bath at 50 CC/0.01 Torr, and the distillate is chromatographed over silica gel eluting with CI1,C12 yield 1.22 g (94%) d.r. synjanti) 92 8. 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

Several detailed studies of reactions of achiral aiiylboronates and chiral aldehydes have been reported4,52 - 57. Diastereofacial selectivity in the reactions of 2-(2-propenyl)- or 2-(2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes with x-methyl branched chiral aldehydes are summarized in Table 252, 53, while results of reactions with a-heteroatom-substituted aldehydes are summarized in Table 34,52d 54- 57. 

Finally, 2-allyl-4,5-tra ,s-diphenyl-l,3-bis(4-methylphenylsulfonyl)-l,3,2-diazaborolidincs have been used74. The 2-propenyl derivative undergoes highly stereoselective reactions with achiral aldehydes (95 - 97% ee) the ( )-2-butenyl derivatives (91-95% ee) and the analogous 2-chloro- and 2-bromo-2-propenyl derivatives (84-99% ee) also give excellent results in reactions with achiral aldehydes. 

The enantioselectivities of the reactions of representative achiral aldehydes and chiral allylboron reagents arc compared in Table 4. A comparison of the enantioselectivities of the (Z )-2-butenyl reagents appears in Table 5, while Table 6 provides a similar summary of the reactions of the (Z)-2-butenyl and 3-methoxy-2-propcnyl reagents. A 3-diphenylamino-2-propenyl reagent was recently reported102. 

Results of the asymmetric 2-propenylborations of several chiral a- and /i-alkoxy aldehydes are presented in Table 11 74a-82 84. These data show that diisopinocampheyl(2-propenyl)borane A and l,3-bis(4-methylphenylsulfonyl)-4,5-diphenyl-2-propenyl-l,3,2-diazaborolidine C exhibit excellent diastereoselectivity in reactions with chiral aldehydes. These results are in complete agreement with the enantioselectivity of these reagents in reactions with achiral aldehydes (Section 1.3.3.3.3.1.4.). In contrast, however, the enantioselectivity of reactions of the tartrate 2-propenylboronate B (and to a lesser extent the tartrate (/i)-2-butenylhoronate)53b is highly... 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

These reagents are not isolated but are used directly in reactions with aldehydes, after generation of ate complexes via the addition of an alkyllithium reagent or pyridine11. 2-(2-Propenyl)-1,3,2-dioxaborolane is also metalated upon treatment with lithium tetramethylpiperidide, but mixtures of a- and y-substitution products are obtained upon treatment of this anion with alkylating agents20. Consequently, this route to a-substituted allylboron compounds appears to be rather limited in scope. 

Alkoxy-substituted allylaluminum reagents diethyl[(Z)-3-methoxy-2-propenyl]- and -[(Z)-3-(l-methoxy-l-methylethyl)-2-propenyl]aluminum have been prepared by treatment of the corresponding alkoxyallyllithiums with diethylaluminum chloride in tetrahydrofuran at — 78 =C4. These reagents provide the syn-diastereomer with 9-11 1 selectivity in reactions with aldehydes at — 78 °C. The reaction of diethyir(Z)-3-methoxy-2-propenyl]a]uminum and acetophenone provided the iy -diastereomer with 4 1 selectivity. 

Trimethyl(l-phenyl-2-propenyl)silane of high enantiomeric excess has also been prepared by asymmetric cross coupling, and reacts with aldehydes to give optically active products in the presence of titanium(IV) chloride. The stereoselectivity of these reactions is consistent with the antiperiplanar process previously outlined75. 

Diallyldialkylstannanes with chiral alkyl substituents on the tin, show variable asymmetric induction in their Lewis acid catalyzed reactions with aldehydes. Using bis-(/f)-2-phenylbutyl-(di-2-propenyl)stannane, enantiomeric excesses of up to 54% were obtained via attack on the / e-face of the aldehyde96. 

Di-2-propenyltin dibromide, available from 2-propenyl bromide and tin powder, reacts with aldehydes in the presence of monosodium (+)-diethyl 2,3-dihydroxybutanedioate to give ho-moallylic alcohols in good yields with 42-71% ee the experimental procedure is relatively straightforward98. 

The best procedure reported to date for the asymmetric allylation of aldehydes using tributyl(2-propenyl)stannane involves the catalyzed addition with the BINOL-TiCl2 complex as catalyst. Good yields and ee s were obtained for both aromatic and aliphatic aldehydes using 20 mol% of the catalyst127. 

Boron trifluoride-diethyl ether complex induced reactions of both (E)- and (Z)-tributyI(3-methoxy-2-propenyl)stannane with aldehydes give. vj-w-products with useful slereoselectivi-... 

Diisopropylamino(dimethyl)silyl]-2-propenyl]lithium adds to aromatic and x-branched aldehydes in the presence of anhydrous zinc chloride with essentially complete anti stereoselectiv-ity3s. as expected from the chair-like pericyclic transition state formed by the ( -intermediate. The addition products are not isolated, but after O-silylation, oxidative desilylation with retention of configuration forms the rmft-diols. 

Several trialkoxy(2-butenyl)zirconium(IV)6,7i 18 and 2-butenylbis(cyclopentadienyl)zirco-nium(IV)18,19 124 complexes have been investigated with respect to the diastereoselectivity on addition to aldehydes. Chlorobis(cyclopentadienyl)-(3-tributylstannyl-2-propenyl)zirconium(IV), prepared by hydrozirconation of tributyl-(l,2-propadienyl)tin, accomplishes the (E)-selective, Wittig-like 1,2-propenylidenation of aldehydes and methyl ketones125. 

Reisse used activated zinc for aqueous Barbier-type reactions.66 Submicromic zinc powder produced by pulsed sono-electroreduction is about three times more effective than the commercial variety. The stereochemical course of the allylation and propargylation of several aldehydes with crotyl and propenyl halides using zinc powder as the... 

---