The following Figure 1 synthesizes a cyclic compound $\mathbf{H}$ with significant ring strain. It is known that the molecular formula of $\mathbf{H}$ is $\mathrm{C_5H_8O}$. Deduce the reaction mechanism and the structures of $\mathbf{F}$, $\mathbf{G}$, and $\mathbf{H}$.

<MultiModal>![Fig. 1, the figure shows a three-step consecutive reaction. The first step reaction is described by SMILES as: CCOC(CC(C)=O)=O>O1CC1>[[F]], where the reaction conditions are EtONa, EtOH. The second step is [[F]]>>[[G]], the reaction condition is HBr. The third step is [[G]]>>[[H]], the reaction condition is base.](/media/uploads/1b1fc339-d92a-41ba-8ddf-7b456a84bfe4.png)</MultiModal>

There are the following statements:

1.  $\mathbf{F}$ contains an ethyl ester group structure
2.  $\mathbf{G}$ contains a carboxyl group structure
3.  $\mathbf{H}$ contains a four-membered ring in the molecule
4.  A substance that is a gas under normal pressure is generated in the entire reaction

The following options that are all correct and have the largest number of correct statements are:
下图1合成了一种具有较大张力的环状化合物 $\mathbf{H}$ ，已知$\mathbf{H}$的分子式为$\mathrm{C_5H_8O}$。推测反应机理和 $\mathbf{F}$、$\mathbf{G}$、$\mathbf{H}$ 的结构。

<MultiModal>![Fig. 1，图中为三步连续反应。第一步反应以SMILES描述为：CCOC(CC(C)=O)=O>O1CC1>[[F]]，其中反应条件为EtONa，EtOH。第二步为[[F]]>>[[G]]，反应条件为HBr。第三步为[[G]]>>[[H]]，反应条件为碱。](/media/uploads/1b1fc339-d92a-41ba-8ddf-7b456a84bfe4.png)</MultiModal>

有以下说法：

1. $\mathbf{F}$内含有乙酯基结构
2. $\mathbf{G}$中含有羧基结构
3. $\mathbf{H}$分子内含有四元环
4. 整个反应中有常压下为气体的物质生成

以下选项中说法全部正确且正确说法数量最多的为：
All other options are incorrect
1
2
3
4
1,2
1,3
1,4
2,3
2,4
3,4
1,2,3
1,2,4
1,3,4
2,3,4
1,2,3,4
其他选项均不正确
1
2
3
4
1,2
1,3
1,4
2,3
2,4
3,4
1,2,3
1,2,4
1,3,4
2,3,4
1,2,3,4
First, according to the final product's molecular formula, it can be found that the number of carbons is reduced by 2 compared to the reactants, indicating that some molecules are substituted during the reaction. At the same time, it only contains one oxygen atom, most likely a carbonyl oxygen.

Ethyl acetoacetate undergoes nucleophilic attack on ethylene oxide under basic conditions, resulting in an alcohol anion. This alkoxide may further undergo an intramolecular transesterification reaction. Considering the extremely small number of carbons in the final product, it is first assumed here that transesterification occurs, removing one molecule of ethanol. The hydroxyl anion substitutes ethanol, yielding $\mathbf{F}$ as shown in Figure 2:

<MultiModal>![Fig. 2，图中分子以SMILES描述为：CC(=O)C1CCOC1=O](/media/uploads/eb532499-ce82-4feb-99c5-f0ec854cb55a.png)</MultiModal>

<Checkpoint>[Nucleophilic addition followed by intramolecular transesterification](1)</Checkpoint>

<Checkpoint>[The structure of $\mathbf{F}$ is described by SMILES as: CC(=O)C1CCOC1=O](1)</Checkpoint>

The structure of $\mathbf{F}$ does not contain ethyl acetate, so statement 1 is incorrect.

In the second step, ester hydrolysis occurs under acidic conditions, yielding a β-keto acid. β-Keto acids are unstable under acidic heating conditions and can undergo decarboxylation. Considering that the final product contains only one oxygen atom, decarboxylation most likely occurred here. The hydroxyl group can be protonated by hydrobromic acid, and at the same time, the bromide ion can nucleophilically attack and substitute the hydroxyl group, yielding $\mathbf{G}$ as shown in Figure 3:

<MultiModal>![Fig. 3，图中分子以SMILES描述为：CC(=O)CCCBr](/media/uploads/94279a75-bc4f-4740-9499-77605a15a598.png)</MultiModal>

<Checkpoint>[β-Keto acid is unstable under acidic heating conditions and undergoes decarboxylation](1)</Checkpoint>

<Checkpoint>[Bromine substitutes the hydroxyl group to obtain $\mathbf{G}$](1)</Checkpoint>

<Checkpoint>[The structure of $\mathbf{G}$ is described by SMILES as: CC(=O)CCCBr](1)</Checkpoint>

The structure of $\mathbf{G}$ does not contain a carboxyl group, so statement 2 is incorrect.

In the third step, under the action of a base, the β-carbon of the carbonyl group can undergo deprotonation, and then nucleophilically substitute the bromine atom. According to the question, the final product $\mathbf{H}$ is a highly strained cyclic compound, so the reaction produces a three-membered ring. The structure of $\mathbf{H}$ is shown in Figure 4:

<MultiModal>![Fig. 4，图中分子以SMILES描述为：CC(=O)C1CC1](/media/uploads/1f1d1dff-f9a1-434d-b173-8e7a7797f4d5.png)</MultiModal>

<Checkpoint>[Nucleophilic substitution of the bromine atom at the β-position of the carbonyl group, yielding $\mathbf{H}$](1)</Checkpoint>

<Checkpoint>[The structure of $\mathbf{H}$ is described by SMILES as: CC(=O)C1CC1](1)</Checkpoint>

Verification confirms that the molecular formula matches the description in the question. The structure of $\mathbf{H}$ is a three-membered ring rather than a four-membered ring, so statement 3 is incorrect.

Looking back at the initial assumptions, if transesterification does not occur in the first step, then the ester group is difficult to hydrolyze under the subsequent acidic and basic conditions. If decarboxylation does not occur in the second step, then the carboxyl group cannot be eliminated under the basic environment in the third step, which does not meet the molecular formula requirements of the question. Overall, the above assumptions are correct.

Decarboxylation in the second step produces $\mathrm{CO_2}$ gas, so statement 4 is correct.

<Checkpoint>[Decarboxylation in the second step produces $\mathrm{CO_2}$ gas](1)</Checkpoint>
首先根据最终产物分子式可以发现，相较于反应物碳数反而降低2，说明反应中有一些分子被取代。同时只含有一个氧原子，最可能是羰基氧。

乙酰乙酸乙酯在碱作用下亲核进攻环氧乙烷，得到醇负离子。该醇盐还可能进一步发生分子内酯交换反应，考虑到最终产物中碳数极少，此处先假设会发生酯交换脱去一分子乙醇。羟基负离子取代乙醇，得到$\mathbf{F}$如图2：

<MultiModal>![Fig. 2，图中分子以SMILES描述为：CC(=O)C1CCOC1=O](/media/uploads/eb532499-ce82-4feb-99c5-f0ec854cb55a.png)</MultiModal>

<Checkpoint>[亲核加成然后发生分子内酯交换反应](1)</Checkpoint>

<Checkpoint>[$\mathbf{F}$结构以SMILES描述为：CC(=O)C1CCOC1=O](1)</Checkpoint>

$\mathbf{F}$结构不含乙酸乙酯，说法1错误。

第二步酸性条件下酯水解，得到β酮酸。β-酮酸在酸性加热条件下不稳定，可发生脱羧反应，考虑到最终产物仅含一个氧原子，此处很可能发生了脱羧。羟基可以被氢溴酸质子化，同时溴离子可以亲核进攻取代羟基，得到$\mathbf{G}$如图3：

<MultiModal>![Fig. 3，图中分子以SMILES描述为：CC(=O)CCCBr](/media/uploads/94279a75-bc4f-4740-9499-77605a15a598.png)</MultiModal>

<Checkpoint>[β-酮酸在酸性加热条件下不稳定，发生脱羧反应](1)</Checkpoint>

<Checkpoint>[溴取代羟基得到$\mathbf{G}$](1)</Checkpoint>

<Checkpoint>[$\mathbf{G}$结构以SMILES描述为：CC(=O)CCCBr](1)</Checkpoint>


$\mathbf{G}$结构不含羧基，说法2错误。

第三步在碱的作用下，羰基α碳可以发生去质子化，然后亲核取代溴原子。根据题目，最终产物$\mathbf{H}$为大张力环状化合物，因此反应生成三元环，$\mathbf{H}$结构如图4：

<MultiModal>![Fig. 4，图中分子以SMILES描述为：CC(=O)C1CC1](/media/uploads/1f1d1dff-f9a1-434d-b173-8e7a7797f4d5.png)</MultiModal>

<Checkpoint>[羰基α位亲核取代溴原子，得到$\mathbf{H}$](1)</Checkpoint>

<Checkpoint>[$\mathbf{H}$结构以SMILES描述为：CC(=O)C1CC1](1)</Checkpoint>

验证分子式符合题中描述。$\mathbf{H}$结构为三元环而非四元环，说法3错误。

回看开头的假设，若第一步不发生酯交换，那么酯基在后面的酸性碱性条件下较难水解。第二步不发生脱羧，那么第三步碱性环境下羧基无法消去，不符合题目分子式要求。综合来看，以上假设正确。

第二步脱羧产生$\mathrm{CO_2}$气体，说法4正确。

<Checkpoint>[第二步脱羧产生$\mathrm{CO_2}$气体](1)</Checkpoint>