Group equivalent scheme

The regiochemistry of the reaction of unsymmetrical allylic esters was addressed by the introduction of a silyl group, equivalent to H, on the carbon of the allylic terminus. Eor example, 25 bearing a BuMe2Si group afforded 26 as the sole product in the three-component coupling. Since the subsequent protodesilylation of 26 selectively formed 27 (Scheme 6.6), this sequential procedure provides a practical method to control the regiochemistry in the substitution of unsymmetrical allylic substrates [13 c]. 

European Commission. 2003. Guidance Document for the Evaluation of the Equivalence of Organic Producer Group Certification Schemes Applied in Developing Countries. European Commission, Brussels. 

Scheme 95 describes in principle the same synthesis technique using a bisphospho-nium salt 562 as the central molecular part of the molecules 563 and 565. Olefination of the ylide generated from 562 with two equivalents of the ketoaldehyde 561 yields ketocarotenoid 563211). Unsymmetrical ketocarotenoid 565 has been synthesized by the Wittig reaction of 562 with a mixture of aldehydes 564 and 511 and subsequent hydrolysis of the acetal protective group 271) (Scheme 95). 

Nitroethylene undergoes rapid cycloaddition to 1,3-dienes subsequent conversion of the nitro to a carbonyl group, e.g. (1) (2) -+ (3), exemplifies its application as a dienophilic ketene equivalent (Scheme... 

P-Sulfonylnitroalkenes or B-sulfinylnitroethylene are excellent dienophiles and correspond to alkyne or nitroalkyne equivalents (Scheme 12)." Bisactivated dienophile (26) reacts with 1,3-dienes in a highly regioselective maimer to give, on subsequent reductive elimination of the nitro and sulfonyl groups, cyclohexadienes, e.g. (16) + (26) (27) (28). p-Sulfinylnitroethylene (30) offers a regioselective... 

With unfruitful results as shown above, we tried Sutherland s approach for constructing the anhydride segment via Homer-Emmons olefination (50) which was finally chosen in our synthetic study (Scheme 18). In this synthesis, differentiation of the three carboxyl group equivalents in the molecule is required. For this purpose, we planned to employ a condensation between triethyl phosphonoacetate and the a-ketoester 101. 

Calculation of tsHf Using Transferable Group Equivalents. The idea that the properties of molecules are the sum of its component atoms and groups has led to the development of schemes by which thermodynamic properties can be calculated as the sum of contributions from all structural units.The most highly developed is that of S. W. Benson and co-workers.The molecule is divided into its component groups. For example, isooctane (which incidentally is the standard for 100 in octane ratings) consists of five C-(C)(H)3 (a), one C-(C)2(H)2 (b), one C-(C)3(H) (c), and one C-(C)4 (d), as labeled on the structure. 

The calculation of A//j is now usually done by computational chemistry, but the success of the group equivalent approaches makes an important point. The properties of groups are very similar from molecule to molecule, similar enough to make additivity schemes workable. However, specific interactions, e.g., nonbonded interactions, that depend on the detailed structure of the molecule are not accounted for. Whenever interactions that are not accounted for by the group equivalents exist, there will be a discrepancy between the calculated and actual properties of the molecule. Analyses such as that of Leroy can provide valuable insights and concepts. In particular, they provide a means for recognizing stabilization effects present in reactants, as demonstrated by the calculations for 1,3-butadiene and dimethoxymethane. 

The 1,3-polyol system is prevalent in a variety of marine natural products. This interesting arrangement of hydroxyl groups can be assembled using aldehydes 574, 575, and 1,3-dithiane as an acyl anion equivalent (Scheme 83) [138]. 

There is also an interesting example of an enantioselective thiourea-catalyzed oxa-Michael reaction using enones as Michael acceptors in which phe-nylboronic acid was employed as hydroxyl anion equivalent (Scheme 4.64) The authors demonstrated that amine bases were able to activate these kinds of reagents by complexation, thus becoming effective reagents for the transfer of the OH group to the Michael acceptor. The reaction had to proceed in an intramolecular way and, for this reason, y-hydroxy-a,(3-unsaturated ketones had to be employed as substrates. In the enantioselective version, 71b was identified as a very efficient catalyst, providing a series of (3,y-dihydroxy ketones in excellent yields and enantioselectivities, after oxidative work-up. The process consists of the initial reaction of the boronic add first with the y-hydroxy... 

Tanikaga and coworkers have reported the addition of nitroalkane anions to a-halo-a,P-unsaturated sulfoxides [72]. Further development of this work led to the use of nitroalkanes as alkyl group equivalents in conjugate addition to a,P-unsaturated sulfoxides [73-75]. Primary or secondary nitroalkanes such as 2-nitropropane (77), with DBU as a non-nucleophilic base, add to a,P-unsaturated sulfoxides including phenyl vinyl sulfoxide (26) to give products such as (78), which can be denitrated to yield (79) (Scheme 5.25). The Michael addition of nitroalkanes, and of diethyl N-acetylaminomalonate, to racemic phenyl vinyl sulfoxide using solid-liquid phase-transfer catalysis in the absence of solvent has also been accomplished [76]. 

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