As shown in Figure 1, the reaction conditions for the first step are: $\mathrm{1)KOH, CH_3OH, reflux, 1h; 2)}$ $\mathbf{a}$ $\mathrm{, K_2CO_3, CH_3CN, 0°C~to~r.t., 3h}$.

The reaction conditions for the second step are: $\mathrm{1) (CF_3CO)_2O, Et_3N, THF, -78°C, 1.5 h; 2)120°C, sealed tube, 24 h; 3)NaOH(5M), r.t., 1h}$.

Propose the reaction mechanism of Figure 1 and the structures of reagent $\mathbf{a}$ and intermediate $\mathbf{A}$.

<MultiModal>![Fig. 1, the figure shows two consecutive steps of reaction, the first step reaction is described by SMILES as: COC1=C2C[C@@]3([H])C[C@H]4COC(N4[C@]3([H])CC2=CC=C1)=O>>[[A]], the second step reaction is described by SMILES as: [[A]]>>CCCN1C[C@H](O)C[C@]2([H])CC3=C(OC)C=CC=C3C[C@]21[H]](/media/uploads/ab10011c-b6a3-4fe4-b101-f29679968b26.png)</MultiModal>

There are the following statements:

1.  Reagent $\mathbf{a}$ can be used in catalytic amount
2.  In the process of generating $\mathbf{A}$, the net number of carbon atoms within the reactant molecule increases by 3
3.  A high-strain intermediate with positive charge is formed during the reaction
4.  A total of 2 carbon-nitrogen single bonds are broken and 2 carbon-nitrogen single bonds are formed during the reaction

The following option contains all correct statements and the largest number of correct statements:
如图1反应，其中第一步反应条件为：$\mathrm{1)KOH, CH_3OH, reflux, 1h; 2)}$ $\mathbf{a}$ $\mathrm{, K_2CO_3, CH_3CN, 0°C~to~r.t., 3h}$。

第二步反应条件为：$\mathrm{1) (CF_3CO)_2O, Et_3N, THF, -78°C, 1.5 h; 2)120°C, sealed tube, 24 h; 3)NaOH(5M), r.t., 1h}$

推测图1反应机理和试剂 $\mathbf{a}$、中间体 $\mathbf{A}$ 的结构。

<MultiModal>![Fig. 1，图中为连续两步反应，第一步反应以SMILES描述为：COC1=C2C[C@@]3([H])C[C@H]4COC(N4[C@]3([H])CC2=CC=C1)=O>>[[A]]，第二步反应以SMILES描述为：[[A]]>>CCCN1C[C@H](O)C[C@]2([H])CC3=C(OC)C=CC=C3C[C@]21[H]](/media/uploads/ab10011c-b6a3-4fe4-b101-f29679968b26.png)</MultiModal>

有以下说法：

1. 试剂 $\mathbf{a}$ 使用催化量即可
2. 生成 $\mathbf{A}$ 的过程中反应物分子内碳原子数净增加了3
3. 反应过程中形成了带正电荷的大张力中间体
4. 反应过程中总共断裂了2个碳氮单键，形成了2个碳氮单键


下列选项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
Observing the structures of the reactants and products, it can be found that the ester group connected to the nitrogen atom disappears, forming a new six-membered ring, and the relative position of the hydroxyl group on the six-membered ring compared to other groups within the molecule has migrated from the reactant to the position of the two fused five-membered ring carbon-nitrogen bonds. These changes suggest that the reactant may have undergone decarboxylation, substitution of the carbon-nitrogen bond by a hydroxyl group, and conversion of the five-membered ring to a six-membered ring.

In the first step, under condition 1), the ester group of the reactant undergoes alkaline hydrolysis, yielding an unstable carboxyl group connected to the amino group, which rapidly undergoes decarboxylation, resulting in a secondary amine and an alcohol hydroxyl group.

<Checkpoint>[Alkaline hydrolysis and decarboxylation of reactant ester group](1)</Checkpoint>

Under condition 2), the intermediate continues to react with reagent $\mathbf{a}$. Based on the product structure and subsequent reaction conditions, there are no conditions in the subsequent reactions to introduce a propyl group at the amino position. Therefore, it is very likely that the role of reagent $\mathbf{a}$ is to introduce a propyl group at the amino position. Condition 2) is a base-catalyzed nucleophilic substitution reaction of the amino group, and reagent $\mathbf{a}$ can be an alkylating reagent such as bromopropane or iodopropane.

<Checkpoint>[Reagent $\mathbf{a}$ can be an alkylating reagent such as bromopropane or iodopropane](1)</Checkpoint>

At least one equivalent of reagent $\mathbf{a}$ must be charged to completely alkylate the reactant, so statement 1 is incorrect.

The structure of intermediate $\mathbf{A}$ is shown in Figure 2:

<MultiModal>![Fig. 2, the molecule in the figure is described by SMILES as: COC1=C2C[C@@]3([H])C[C@H](N(CCC)[C@]3([H])CC2=CC=C1)CO](/media/uploads/dffbd413-82fd-4faa-8afa-dbeed36ddbe5.png)</MultiModal>

One molecule of carboxyl group is removed and one molecule of propyl group is introduced during the formation of $\mathbf{A}$, resulting in a net increase of 2 carbons, so statement 2 is incorrect.

<Checkpoint>[The structure of $\mathbf{A}$ is described by SMILES as: COC1=C2C[C@@]3([H])C[C@H](N(CCC)[C@]3([H])CC2=CC=C1)CO](1)</Checkpoint>

Condition 1) in the second step is a typical trifluoroacetylation condition, which can acylate the hydroxyl group to activate it. Condition 2) raises the system temperature from -78°C to 120°C for the reaction, and the activated trifluoroacetate ion leaves. Considering that the final product undergoes a ring expansion from a five-membered ring to a six-membered ring, this step is likely the nucleophilic attack of the nitrogen atom on the five-membered ring on the carbon where the trifluoroacetate group is located, replacing the trifluoroacetate group to obtain an unstable azatrimethylenimine intermediate as shown in Figure 3:

<MultiModal>![Fig. 3, the molecule in the figure is described by SMILES as: COC1=C2C[C@@]3([H])C[C@H]4C[N+]4(CCC)[C@]3([H])CC2=CC=C1](/media/uploads/54731f56-5378-4c79-b69a-4536acc8ff2f.png)</MultiModal>

<Checkpoint>[The hydroxyl group is activated and substituted by an amino group, forming a three-membered ring aziridinium ion intermediate](1)</Checkpoint>

This positively charged intermediate has high ring strain and is unstable, so statement 3 is correct.

In 3), under concentrated alkali conditions, hydroxide replaces the aziridinium ion, unraveling the unstable three-membered ring structure, resulting in the ring-expanded final product.

<Checkpoint>[Hydroxyl group replaces amino group, ring-opening to obtain a six-membered ring system](1)</Checkpoint>

During the entire reaction process, decarboxylation breaks one carbon-nitrogen bond, alkylation forms one carbon-nitrogen bond, formation of the three-membered ring forms one carbon-nitrogen bond, and unraveling the three-membered ring breaks one carbon-nitrogen bond, so statement 4 is correct.
观察反应物和产物的结构，可以发现与氮原子连接的酯基消失，形成了一个新的六元环，且该六元环上羟基与分子内其他基团的的相对位置相比反应物迁移到了两个五元并环原碳氮键位置。以上变化暗示反应物可能经过了脱羧、碳氮键被羟基取代、以及五元环向六元环的转化。

第一步1)条件下反应物酯基碱性水解，得到不稳定与氨基连接的羧基，快速发生脱羧，得到二级胺和一个醇羟基。

<Checkpoint>[反应物酯基碱性水解，脱羧](1)</Checkpoint>

在2) 条件下，中间体继续与试剂$\mathbf{a}$反应。根据产物结构和后续的反应条件，后续反应中并没有条件可以在氨基位引入丙基，因此很有可能试剂$\mathbf{a}$的作用即为在氨基位引入丙基。2)条件为碱性催化氨基的亲核取代反应，试剂$\mathbf{a}$可以是溴丙烷、碘丙烷等烷基化试剂。

<Checkpoint>[试剂$\mathbf{a}$可以是溴丙烷、碘丙烷等烷基化试剂](1)</Checkpoint>

试剂$\mathbf{a}$至少投料一当量才能完全烷基化反应物，说法1错误。

得到中间体$\mathbf{A}$结构如图2：

<MultiModal>![Fig. 2，图中分子以SMILES描述为：COC1=C2C[C@@]3([H])C[C@H](N(CCC)[C@]3([H])CC2=CC=C1)CO](/media/uploads/dffbd413-82fd-4faa-8afa-dbeed36ddbe5.png)</MultiModal>

生成$\mathbf{A}$的过程中脱去了一分子羧基，引入了一分子丙基，净碳数增加2，说法2错误。

<Checkpoint>[$\mathbf{A}$结构以SMILES描述为：COC1=C2C[C@@]3([H])C[C@H](N(CCC)[C@]3([H])CC2=CC=C1)CO](1)</Checkpoint>

第二步条件1)为典型三氟乙酰化条件，可以酰化羟基使其活化。条件2）将体系从-78°C升温至120°C反应，被活化的三氟乙酸根离去。考虑到最终产物发生了五元环到六元环的扩环，该步很可能是五元环上氮原子亲核进攻三氟乙酸根所在碳，取代三氟乙酸根得到一个不稳定的氮杂三元环中间体如图3：

<MultiModal>![Fig. 3，图中分子以SMILES描述为：COC1=C2C[C@@]3([H])C[C@H]4C[N+]4(CCC)[C@]3([H])CC2=CC=C1](/media/uploads/54731f56-5378-4c79-b69a-4536acc8ff2f.png)</MultiModal>

<Checkpoint>[羟基被活化后被氨基取代，形成三元环鎓离子中间体，以SMILES描述为：COC1=C2C[C@@]3([H])C[C@H]4C[N+]4(CCC)[C@]3([H])CC2=CC=C1](1)</Checkpoint>

该带正电荷中间体张力大不稳定，说法3正确。

3)中浓碱下氢氧根取代氮鎓离子，解开不稳定的三元环结构，得到扩环后的最终产物。

<Checkpoint>[羟基取代氨基，开环得到六元环体系](1)</Checkpoint>

整个反应过程中脱羧断开一根碳氮键，烷基化形成一根碳氮键，形成三元环形成一根碳氮键，解开三元环断开一根碳氮键，说法4正确。