Butadiene moieties

The domino process probably involves the chiral enamine intermediate 2-817 formed by reaction of ketone 2-813 with 2-815. With regard to the subsequent cy-doaddition step of 2-817 with the Knoevenagel condensation product 2-816, it is interesting to note that only a normal Diels-Alder process operates with the 1,3-bu-tadiene moiety in 2-817 and not a hetero-Diels-Alder reaction with the 1-oxa-l,3-butadiene moiety in 2-816. The formed spirocydic ketones 2-818/2-819 can be used in natural products synthesis and in medidnal chemistry [410]. They have also been used in the preparation of exotic amino adds these were used to modify the physical properties and biological activities of peptides, peptidomimetics, and proteins... 

Dailey and coworkers [6] extended the studies of Prinzbach using 4-1 as substrate. These authors found that, by employing dicyanoacetylene 4-15 in the reaction with 4-1 the domino adduct 4-16 but not 4-17, as expected, is the main product (Scheme 4.4). In the formation of 4-16 one of the two 1,3-butadiene moieties in 4-1 has reacted with the dienophile 4-14 from the inside, followed by a second [4+2] cycloaddition of the formed dicyanoethene moiety. 4-17 is observed as a side product here, in the first step, the dienophile reacts from the outside, while in the second step the other formed dienophile moiety undergoes a cycloaddition with the second 1,3-bu-tadiene moiety. This mode of action is actually favored in the reaction of all other dienophiles employed, due to their larger size when compared to 4-15. 

Alternative pathways for coupling of two cA-butadiene or two trans-butadiene moieties, however, are unfeasible on considerations similar to those discussed for the Cg-channel (cf. Section 6.1). 

The planar C2h and C2V geometries of the 1,3-butadiene moiety are achiral structures and obviously they cannot show optical activity (i.e. ORD and CD). This has, of course, a spectroscopic origin. The optical activity of a transition Pq — Pi is determined by its Rotational Strength (R)1 defined as the scalar product... 

The main interest of this molecule resides in the fact that the principal source of rotational strength of the it - it lowest energy transition has been attributed40 to the twist of one of the two double bonds (a = —136°, as in fraws-cyclooctene) rather than to the twist of the 1,3-butadiene moiety (6 = +50.2°)... 

FIGURE 22. Top Labels of the four localized basis w-orbitals of [4.4]spirononatetraene 247 and Newman projection defining their relative phases. Middle Newman projections of the four linear combinations ld2(tr), b <7T) and 1 c(jt). defined in equation 59. Bottom Correlation diagram showing the splitting due to spiroconjugation between the butadiene moieties in 247... 

By using the conventional bond formula (V) for the metal butadiene moiety, we may describe this cyclo-addition process as an orbitally allowed 7r2, + 7r2a + a2, pericyclic reaction (160). (See Fig. 15 for an illustration of the topology of the orbital interactions.)... 

Iantherans A (516) and B (517) are dimeric tetrabrominated benzofuran derivatives that were isolated from an Australian Ianthella species. The structures were determined by spectroscopic and chemical methods. Iantheran A (516) includes a (Z,Z)-1,3-butadiene moiety, whereas iantheran B (517) is the geometric isomer possessing a (Z, )- 1,3-butadiene moiety. Both compounds were Na+/K+-ATPase inhibitors [434,435]. Ianthesines C (518) and D (519) showed potent Na+/K+-... 

The X-ray crystal structure of the 1 -tosyl-1H-1,2-diazepine (10) shows a boat conformation with N(l) at the prow (Figure 1) (72T581). Double bonds are clearly localized at N(2)—C(3) 1.255 A, C(4)—C(5) 1.326 A, and C(6)—C(7) 1.333 A, and the imine bond is isolated, whereas bond delocalization is present in the butadiene-like part C(4)—C(5)—C(6)—C(7). In contrast, when the diazepine is complexed with Fe(CO)3, for example (11), the iron atom is bound to the butadiene moiety C(4) to C(7), as also occurs in the lf/-azepine and l-methoxycarbonyl-l//-azepine complexes the N(l) atom adopts a planar sp2 configuration and the seven-membered ring consists of the two planar parts bent along the C(4)... C(7) line by 140° (70AG(E)958). 

Hz) are very close to Jsi-m = 192-225 Hz in [HSi(OR)4] K+ and are consistent with a trigonal-bipyramidal structure in which one hydrogen and two carbons of the butadiene moiety occupy the equatorial positions. 

Early in the development of liposomes, it was recognized that their plasma instability could be a serious detriment in certain applications. Consequently, there were efforts to Lrst incorporate polymerizable lipids into the liposomal bilayers, and then initiate polymerization by, for example, photolysis, to form interchain crosslinks to stabilize the bilayer. The most commonly used polymerizable lipids have been PCs-containing diacetylene or butadiene moieties in the tailgroups... 

The intermediate 79 obtained by the Knoevenagel condensation of 80 and 81 contains a 1-oxa-l,3-butadiene as well as a normal 1,3-butadiene moiety thus both a hetero-Diels-Alder and a normal Diels-Alder reaction is possible. The dom-... 

The formation of the three diastereomers 134, 144 and 145 in the twofold-domino processes of (S)-136, 35 and 137 is primarily due to the flexibility of the 1-oxa-l,3-butadiene moiety in 139, which can exist in the two different conformations. To improve the induced diastereoselectivity in the cycloaddition, benzyl-tetrahydoisoquinolineacetaldehyde 154 was synthesized, which contains a methoxy group at the 8-position [45]. The Knoevenagel condensation of 154 and Meldrum s... 

Some nitrogen ligands can stabilize the Cp M(butadiene) moiety considerably. Sita et al. have used the stabilizing ability of an acetamidinato ligand to induce a novel synthetic entry into Cp M(butadiene)L chemistry. When the [di-( -butyl)Zr(acetamidinato)Cp ] complex 60 was heated, it eliminated H2 along with one equivalent of //-butane to yield the butadiene complex 62 (probably via the intermediate 61)66 (Scheme 20). 

In this study it is demonstrated that polymerization rates as well as molar masses decrease with increasing substitution at the butadiene moiety. It is remarkable that in all cases except for E,E-2,4-hexadiene high cis- 1,4-contents are obtained. Most of the polymers based on substituted butadienes of the type CH2 = CH - CH = CHR are highly crystalline and highly isotactic. 

Fig. 15.14. Evidence for stereoselectivity and stereospecificity with regard to the butadiene moiety in a pair of Diels-Alder reactions. The cis,trans-l,4-disubstituted 1,3-butadiene forms cyclohexene with a trans arrangement of the methyl groups. The trans,trans-1,4-disubstituted 1,3-butadiene forms cyclohexene with ris-methyl groups.

Another all-carbon Diels-Alder reaction is proposed for the biosynthesis of the indole alkaloids tabersonine 1-6 and catharanthine 1-7 of the Aspidosperma and Iboga family [28-31]. The compounds are formed via strictosidine 1-3, the first nitrogen-containing precursor of the monoterpenoid indole alkaloids, and stemmadenine 1-4, which is cleaved to give the proposed intermediate dehy-drosecodine 1-5 with an acrylate and a 1,3-butadiene moiety (Scheme 1-1). 

---