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Meldrum's acid (1, isopropylidene malonate or 2,2-dimethyl- 4,6-dioxo-1,3-dioxane), was first synthesized by Meldrum in 19081 and the structure was incorrectly assigned as the b-lactone of b-hydroxyisopropylmalonic acid (b,b-dimethyl-b-propiolactone-a-carboxylic acid).2 It was only 40 years later that Davidson and Bernhard assigned the correct structure.2 Meldrum’s acid has attracted considerable attention due to its high acidity (pKa = 4.97)3 and rigid cyclic structure. Acylated derivatives (synthetic equivalents of mixed ketenes) readily undergo alcoholysis to give b-keto esters.4 Alkylidene derivatives are strong electrophiles and can undergo Diels-Alder reactions with high diasteroselectivity.5 This versatile tool is a key intermediate for a large number of important building blocks and there remains much to discover. (A) Substituted g-pyrones 6 can be used in the synthesis of polyacetate- and spiroketal-containing natural products and as cycloaddition substrates for the construction of polycyclic systems. Acylated Meldrum’s acid 3 reacts with a vinyl ether 4 with loss of acetone and ring closure to the pyrone ring followed by decarboxylation. The g-pyrone can then be obtained refluxing with a PTSA– H2O mixture (40–85% yield). A variety of acid chlorides and vinyl ethers can be used to prepare mono-, di- and trisubstituted pyrones (B) Optically active b-lactams 9 are obtained in excellent yields (up to 93%) and with complete stereoselectivity from Meldrum’s acid derivatives 7 and D2-thiazolines 8.7 (C) 2-Alkyl and 2-aryl 4-quinolones 14 can be prepared starting from Meldrum’s acid 1 via their derivatives bisalkylthiolydine 10 (32–77% yield) and alkyl- and arylthioalkylidene 11 (52–90% yield). Further reaction with arylamines 12 gives the arylaminoalkylidene derivatives 13 (54–87% yield) which provide the quinolone products upon heating (60–96% yield).8 (D) The reaction of Meldrum’s acid and its 5-substituted derivatives 15 with conjugated azoalkenes 16, in the presence of triethylamine, produces the corresponding hydrazones 17 (71–98% yield) via 1,4-conjugate addition. The hydrazone products then undergo decarboxylative alcoholysis and simultaneous cyclization to give 3-unsubstituted and 3-monosubstituted 1-amino-1H-pyrrol-2- (3H)-ones 18 (55–96% yield).9 (E) C-Dialkylation of Meldrum’s acid and 5-benzyl Meldrum’s acid can be achieved through Mitsunobu dehydration using allylic and arylmethyl alcohols as alkylating agents. With primary alcohols, the alkylations always take place at the carbon bearing the hydroxyl group. For secondary alcohols, a catalytic amount of Pd(0) must be added to enhance the regioselectivity of C-alkylation over O-alkylation. Using (Ph3P4)Pd(0) as a catalyst, the reaction occurs specifically at the less hindered carbon, and the configuration of the resulting double bound is always trans.10 (F) 5-Substituted 3-isoxazolols 27 can be synthesized in a threestep procedure starting from Meldrum’s acid. After conversion into the acylated derivative 24 (74–100% yield), aminolysis with N,O-bis(tert-butoxycarbonyl)hydroxylamine (25) leads to the N,O-diBoc b-keto hydroxamic acid 26 (53–91% yield). Treatment with hydrochloric acid promotes cyclization to the desired product (76–99% yield).11 (G) The pyrolysis of Meldrum’s acid derivatives in solution and in gas phase takes place by loss of acetone and carbon dioxide to provide ketene intermediates. In particular, methylene derivatives often provide methyleneketenes, which act as substrates for internal hydrogen transfer, leading to cyclization reactions. These cyclization reactions are used for the efficient preparation of a diverse range of cyclic compounds such as quinolinones, 3-hydroxythiophenes, naphthols, azepin-3(2H)-ones or pyrrolizin-3-ones, initiated respectively by 1,3-, 1,4-, 1,5-, 1,6 or 1,7-prototropic shifts. |
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tanghuai
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bohaizmr0(金币+2,VIP+0):谢谢你了！ 3-28 19:01
bohaizmr0(金币+2,VIP+0):谢谢你了！ 3-28 19:01
| 梅尔德拉姆酸(1-异丙叉异丙叉或2,2-二甲基-4,6-二氧基-1,3-二噁烷)在19081年首次由Meldrum合成，并且把其错误的认为是β-内酯(b,b-二甲基-b丙内酯-a-羧酸)。四十年后，Davidson和Bernhard就纠正了这一结构。梅尔德拉姆酸由于其强酸性(Pka=4.97)和刚性结构引起了广泛关注。酰化衍生物(合成的乙烯酮混合物)很容易醇解成b-氧代酯。亚烷基衍生物有很强的亲核力能发生D-A反应且具有高非对映选择性。这是用于许多重要结构单元的重要中间体，且需要进一步的研究。 |
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jerry.lubo
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tanghuai
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