Efficient Syntheses of Vitamin K Chain-Shortened Acid Metabolites

Vitamin K is an umbrella term describing a family of molecules containing the 2-methyl-1,4,-naphthoquinone core. The most widely studied are phylloquinone (Vitamin K1 and a form of Vitamin K2, menaquinone-4 (MK-4), that differ in the degree of unsaturation along the C20-phytyl chain (Scheme 1). Whereas Vitamin K1 primarily functions as a regulator of hemostasis, MK-4 appears important for bone health;1 it is also implicated in vascular calcification2 and it regulates ATP production in mitochondria.3 While the physiological roles of Vitamin K continue to be evaluated, little information regarding metabolism and excretion of Vitamin K metabolites is available. In humans, Vitamin K1 and MK-4 are metabolized to two glucuronide conjugates of chain-shortened carboxylic acid metabolites, referred to as vitamin K acid 1 (1) and vitamin K acid 2 (2) (Figure 1).4–6 In order to quantify Vitamin K acid metabolites in biological matrices to understand Vitamin K metabolism, authentic analytical standards are required. Scheme 1 Terminal oxidation products of vitamin K metabolism Reported syntheses of vitamin K acid 1 (1) are incompletely described and required several laborious steps, some of which result in the partial isomerization of the side chain 2’,3’-double bond.7–10 The menadione-cyclopentadiene adduct 3 previously alkylated with C20-phytyl allylic halides11 to obtain Vitamin K1 was used as starting material for 1 in our synthesis (Scheme 2). Alkylation with geranyl bromide gave trans-geranyl adduct 4 in 79% yield. The cyclopentadiene protecting group was removed by heating 4 in AcOH with catalytic dodecyl-trimethylammonium bromide to produce 5 in 97% yield. The previously reported regioselective epoxidation of geraniol derivatives, followed by oxidative cleavage with periodic acid to yield corresponding aldehydes12 was utilized to obtain aldehyde 7. Epoxidation (mCPBA) of alkene 5 afforded 6 (67% yield), and subsequent periodic acid oxidation gave aldehyde 7 (65% yield). Aldehyde 7 was oxidized with potassium peroxymonosulfate13 yielding trans-vitamin K acid 1 (1) (70% yield). The trans configuration of (1) was confirmed by a 2D-NOESY experiment (supplemental), which did not show a strong NOE cross-peak between the vinyl proton at C2’ and the vinyl methyl group at C3’. Scheme 2 Improved Synthesis of vitamin K acid 1 (1) Previously reported syntheses of vitamin K acid 2 (2) required several steps with poor yields of intermediates.7,10 New methodology was utilized to synthesize vitamin K acid 2 (2) more efficiently (Scheme 3). Menadione-cyclopentadiene adduct 3 was alkylated with dimethylallyl chloride to afford compound 8 in 82% yield. Following deprotection, 9 was subjected to allylic oxidation14 with SeO2 to give allylic alcohol 10 (57% yield). Subsequent reduction of the allylic alcohol by ruthenium-catalyzed transfer hydrogenation15 afforded saturated alcohol 11 in 17% yield. Multiple attempts at improving the percent conversion and yield of 11 were made by experimenting with several ruthenium catalysts. Initially, transfer hydrogenation was attempted with [{RuCl(µ-Cl)(η6-C6Me6)}2] and Cs2CO3, but the double bond remained intact as evidenced by the corresponding 1H NMR triplet signal at C3’. Subsequently, saturation of 10 with [Ru(cod)Cl2]n and potassium hydroxide16 resulted in a 1:1 mixture of 10 and 11 (34% yield). Lastly, we tried [{RuCl(µ-Cl)(η6-para-cymene)}2] with Cs2CO3 and the 1H NMR showed 70% conversion. After increasing the catalyst:base ratio to 25:50 mol%, respectively, pure 11 was isolated in 17% yield. Naphthoquinone containing molecules such as Vitamin K and menadione are notoriously unstable when exposed to heat, light, alkali, and reducing conditions. In each of the transfer hydrogenation reactions attempted, greater than 60% of the starting material was not recovered, which we attribute to probable degradation of the naphthoquinone. To complete the synthesis, 11 was oxidized to the carboxylic acid with periodic acid and pyridinium chlorochromate17 to afford vitamin K acid 2 (2) (84% yield). Scheme 3 Improved Synthesis of vitamin K acid 2 (2) The syntheses described herein provide facile routes to vitamin K acid 1 and 2 utilizing established reaction procedures and inexpensive starting materials. Overall yields for vitamin K acid 1 and 2 were 23% and 5%, respectively. The limiting step in the vitamin K acid 2 synthesis is the saturation of 11, which is mainly the result of naphthoquinone instability under the experimental conditions described. During the synthesis of vitamin K acid 1, the trans configuration of the 2’,3’-double bond was retained resulting in the production all trans-vitamin K acid 1 without the need for geometric isomer recrystallization as previously described7. Overall, the synthesized metabolites will serve as authentic standards for our future investigations of vitamin K metabolism.